Chapter 13 - Viruses, Viroids, and Prions (Oct. 13)

Virus (plural: Viruses)

A non-living infectious agent made up of genetic material (DNA or RNA) enclosed in a protein coat, and sometimes surrounded by a lipid envelope. Viruses cannot reproduce or carry out metabolism on their own; they must infect a living host cell and hijack its machinery to replicate. Viruses can infect all forms of life, including animals, plants, fungi, and bacteria (in which case they’re called bacteriophages).

Cell membrane

(Also known as the plasma membrane) a flexible, semi-permeable barrier that surrounds the cytoplasm of a cell, separating its internal environment from the external surroundings. Composed primarily of a phospholipid bilayer with embedded proteins, it regulates the movement of substances in and out of the cell, maintaining homeostasis. The membrane also plays roles in cell signaling, structural support, and interaction with other cells. Its selective permeability allows essential nutrients to enter, waste products to exit, and communication signals to be transmitted, making it vital for cellular function and survival.

Deoxyribonucleic acid (DNA)

 A double-stranded nucleic acid that carries the genetic instructions used in the development, functioning, growth, and reproduction of all known living organisms and many viruses. It is composed of nucleotides containing a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (adenine, thymine, cytosine, or guanine). The sequence of these bases encodes hereditary information, and the complementary base pairing (A–T, G–C) enables DNA replication and transmission of genetic material.

Ribonucleic acid (RNA)

A single-stranded nucleic acid made of nucleotides containing the sugar ribose, a phosphate group, and one of four nitrogenous bases: adenine (A), uracil (U), cytosine (C), or guanine (G). RNA plays key roles in the expression of genetic information, including acting as a messenger (mRNA) between DNA and proteins, forming structural and catalytic components (rRNA and ribozymes), and carrying amino acids during protein synthesis (tRNA).

Capsid

(Also known as a protein coat) the protein shell that encases and protects the genetic material of a virus. Composed of repeating subunits called capsomeres, the capsid provides structural integrity and plays a crucial role in the virus’s ability to attach to and enter host cells. Its shape can vary — commonly helical, icosahedral, or complex — depending on the virus type. In bacteriophages, the capsid also assists in injecting viral DNA into bacterial cells. By shielding the viral genome and facilitating infection, the capsid is essential for viral survival and transmission.

Lipid

A broad class of naturally occurring organic molecules that are insoluble in water but soluble in nonpolar solvents (like oils or alcohols), due to their largely hydrophobic structure. They include fats, oils, phospholipids, steroids, and waxes, and are made mostly of carbon, hydrogen, and a small amount of oxygen. Lipids play essential roles in living organisms, such as serving as a major energy storage source, forming cell membranes (phospholipid bilayers), and acting as signaling molecules (like steroid hormones).

Single host species

A specific organism that serves as the exclusive biological host for a particular parasite, pathogen, or symbiont. In this relationship, the microbe or virus can only infect, replicate, or complete its life cycle within that one species. This host specificity can result from molecular compatibility, immune evasion strategies, or ecological constraints. For example, Mycobacterium leprae, the bacterium that causes leprosy, primarily infects humans and armadillos, showing a narrow host range. In contrast to broad host range organisms, those restricted to a single host species are more vulnerable to host population changes but may evolve highly specialized interactions.

Fimbria (plural: Fimbriae)

Small, hair-like projections found in both human anatomy and microbiology, serving distinct functions in each context. In human anatomy, fimbriae are the finger-like extensions at the end of the fallopian tubes that help guide the ovulated egg from the ovary into the tube. In microbiology, fimbriae are short, bristle-like appendages on the surface of many bacteria that enable them to adhere to surfaces, host cells, or other bacteria, playing a crucial role in colonization and infection. Though structurally different, both types of fimbriae are essential for facilitating contact and movement in their respective systems.

Nucleic acid

Large biomolecules—DNA and RNA—that store, transmit, and express genetic information in living organisms and viruses. They are composed of chains of nucleotides, each containing a nitrogenous base, a sugar (either ribose or deoxyribose), and a phosphate group. DNA holds the instructions for cellular structure and function, while RNA helps carry out those instructions, especially in protein synthesis. These molecules act as the blueprint and messenger of life, guiding heredity, development, and cellular activity across generations.

Single-stranded

Refers to a molecular structure composed of only one linear chain of nucleotides, rather than the typical double-stranded configuration found in DNA. In biology, this term often describes single-stranded DNA (ssDNA) or single-stranded RNA (ssRNA), where the nucleotides are not paired with a complementary strand. Single-stranded molecules are more flexible and reactive, and they play key roles in processes like replication, transcription, and viral infection. For example, many viruses carry single-stranded genomes, which can be either positive-sense (directly readable by ribosomes) or negative-sense (requiring conversion before translation).

Double-stranded

Refers to a molecular structure composed of two complementary chains of nucleotides that are bound together by base pairing. In DNA, these strands form a stable double helix, with adenine pairing with thymine and cytosine pairing with guanine. This configuration provides structural integrity, protects genetic information, and enables accurate replication and repair. Double-stranded molecules are less reactive than single-stranded ones and are the standard form for most cellular DNA, while some viruses also carry double-stranded RNA genomes.

Linear

Describes a structure or arrangement that follows a straight, unbranched path. In molecular biology, it often refers to DNA or RNA molecules whose nucleotide chains have distinct ends — a 5′ (five-prime) and a 3′ (three-prime) end — rather than forming a closed loop like circular DNA. Linear DNA is typical of eukaryotic chromosomes and some viruses, and its exposed ends influence replication, stability, and vulnerability to degradation. The term can also apply to processes or patterns that progress in a direct, sequential manner without looping or branching.

Circular

Refers to a molecular structure that forms a closed loop, with no free ends. In biology, this term often describes circular DNA, where the nucleotide chain connects end-to-end, creating a continuous ring. This configuration is typical of bacterial chromosomes and plasmids, as well as some viral genomes. Circular DNA is generally more stable than linear DNA and less susceptible to degradation by exonucleases. Its topology influences replication, gene expression, and mobility, especially in processes like conjugation and transformation.

Capsomere

One of the repeating protein subunits that assemble to form the capsid, or protein coat, of a virus. These subunits fit together in precise geometric patterns — often helical or icosahedral — to create a protective shell around the viral genetic material. The arrangement and number of capsomeres determine the shape and stability of the virus. Each capsomere may be composed of one or more protein molecules, and their interactions are crucial for viral assembly, host recognition, and infectivity.

Nucleocapsid

One of the repeating protein subunits that assemble to form the capsid, or protein coat, of a virus. These subunits fit together in precise geometric patterns — often helical or icosahedral — to create a protective shell around the viral genetic material. The arrangement and number of capsomeres determine the shape and stability of the virus. Each capsomere may be composed of one or more protein molecules, and their interactions are crucial for viral assembly, host recognition, and infectivity.

Envelope

An outer lipid membrane that surrounds the nucleocapsid of certain viruses, forming an additional protective and functional layer. This envelope is typically derived from the host cell’s membrane during viral replication and budding, and it contains embedded viral proteins that aid in host recognition and entry. Enveloped viruses, such as influenza and HIV, use this layer to fuse with host cell membranes, facilitating infection. While the envelope enhances infectivity and immune evasion, it also makes the virus more sensitive to environmental conditions like heat, drying, and detergents.

Protein spike

A protruding protein structure found on the surface of certain viruses, such as coronaviruses and influenza viruses, that plays a critical role in host cell recognition and entry. These spikes bind to specific receptors on the host cell membrane, triggering fusion or endocytosis that allows the viral genome to enter the cell. Protein spikes are often the target of immune responses and vaccines because they are exposed and essential for infectivity. Their shape and binding specificity determine the virus’s host range and tissue tropism.

Morphology

The study of the form, structure, and physical characteristics of organisms or their specific parts. This includes aspects like shape, size, arrangement, and external features — whether at the level of whole organisms (e.g., body plans, appendages) or microscopic structures (e.g., cells, viruses, tissues). For example, viral morphology examines capsid shape and envelope presence, while bacterial morphology categorizes cells as cocci (spherical), bacilli (rod-shaped), or spirilla (spiral). Morphology helps classify organisms, understand function, and trace evolutionary relationships.

Polyhedral

Describes a geometric shape or structure composed of flat faces and straight edges, typically forming a three-dimensional solid with multiple polygonal surfaces. In virology, a polyhedral virus has a capsid shaped like a polyhedron — most commonly an icosahedron, which has 20 triangular faces and offers both symmetry and stability. This shape allows efficient packaging of genetic material and is common among non-enveloped viruses. The term emphasizes the angular, multi-faced architecture that contrasts with helical or complex viral forms.

Helical

Describes a spiral-shaped structure, resembling a corkscrew or spring. In virology, a helical virus has a capsid formed by protein subunits arranged around the viral nucleic acid in a continuous spiral, creating a rod-like or filamentous shape. This configuration allows the capsid to accommodate genomes of varying lengths and is commonly seen in RNA viruses like the tobacco mosaic virus. The helical structure provides both flexibility and protection, influencing how the virus assembles and infects host cells.

Enveloped

Describes a virus that possesses an outer lipid membrane surrounding its nucleocapsid. This envelope is typically acquired from the host cell’s membrane during viral replication or release and is embedded with viral glycoproteins (often called protein spikes) that facilitate attachment and entry into new host cells. Enveloped viruses — such as HIV, influenza, and herpesviruses — tend to be more sensitive to environmental factors like heat, drying, and detergents, making them less stable outside the host compared to non-enveloped viruses. The envelope enhances infectivity but also introduces vulnerability.

Complex

Describes a virus with a structure that doesn’t fit neatly into the standard helical or polyhedral categories. These viruses often have multiple components or specialized features, such as tails, fibers, or layered envelopes. A classic example is the bacteriophage, which has an icosahedral head that stores genetic material and a helical tail used to inject DNA into bacterial cells. The term emphasizes architectural intricacy and functional specialization, reflecting how these viruses evolved to interact with specific hosts in precise ways.

Family

A taxonomic rank used to classify a group of related organisms that share common characteristics and are more closely related to each other than to organisms in other families. It sits between order and genus in the hierarchy of biological classification. For example, in virology, the family Herpesviridae includes viruses like HSV-1 and HSV-2, which share structural and genetic traits. In zoology, the family Felidae includes cats like lions, tigers, and domestic cats. The concept helps organize biodiversity into nested categories based on evolutionary relationships.

-viridae

A standardized suffix used in virus taxonomy to indicate a virus family, following conventions set by the International Committee on Taxonomy of Viruses (ICTV). It groups together viruses that share structural, genetic, and evolutionary traits, helping scientists classify and study them systematically. For example, Herpesviridae includes herpes viruses, Coronaviridae includes coronaviruses like SARS-CoV and MERS-CoV, and Poxviridae includes viruses such as smallpox. This naming system parallels how -idae is used for animal families in zoology, reinforcing hierarchical relationships in biological classification.

Genus

A taxonomic rank in biological classification that groups together species that are closely related and share common characteristics. It sits above species and below family in the hierarchy. For example, in humans, the genus is Homo, which includes Homo sapiens and extinct relatives like Homo neanderthalensis. In virology, a genus like Simplexvirus includes herpesviruses that infect mammals. The genus name is always capitalized and used as the first part of a species’ binomial name, helping organize biodiversity by evolutionary lineage and structural traits.

-virus

Used to designate a genus of viruses, grouping together species that share genetic, structural, and evolutionary traits. This naming convention, established by the International Committee on Taxonomy of Viruses (ICTV), helps organize viruses below the family level (which uses the suffix -viridae) and reflects biological similarities within each genus. Examples include Betacoronavirus, which includes SARS-CoV and MERS-CoV, and Simplexvirus, which includes herpes simplex viruses. The -virus suffix acts as a linguistic marker for viral identity and classification.

Species

The most specific level of organism classification, defined as a group of individuals that can interbreed and produce fertile offspring under natural conditions. Members of a species share common genetic traits, morphology, and behavior, and are reproductively isolated from other such groups. This concept helps scientists organize biodiversity and trace evolutionary relationships. For example, Homo sapiens is the species name for modern humans, distinguished by unique cognitive and anatomical features. Species are the narrative units of life’s diversity—each one a distinct chapter in the evolutionary story.

Herpesviridae

A family of double-stranded DNA viruses known for their ability to establish latent infections in host cells, often reactivating later in life. Members of this family infect a wide range of animals, including humans, and are classified into three subfamilies—Alpha-, Beta-, and Gammaherpesvirinae—based on their biological behavior and host range. Notable human pathogens include Herpes Simplex Virus 1 and 2 (HSV-1, HSV-2), Varicella-Zoster Virus (VZV), which causes chickenpox and shingles, and Epstein-Barr Virus (EBV), linked to mononucleosis and certain cancers. Herpesviridae viruses are enveloped, often lifelong, and biologically stealthy—masters of persistence and immune evasion.

Simplexvirus

A genus within the Herpesviridae family that includes Herpes Simplex Virus type 1 (HSV-1) and type 2 (HSV-2), both of which infect mammals, especially humans. These viruses are enveloped, double-stranded DNA viruses known for establishing latent infections in nerve cells, with periodic reactivation. HSV-1 typically causes oral herpes (cold sores), while HSV-2 is more often associated with genital herpes, though both can infect either site. Simplexviruses are biologically stealthy, using latency and immune evasion to persist in the host over a lifetime.

Humanalpha2

The official species name for Herpes Simplex Virus type 2 (HSV-2), classified under the genus Simplexvirus in the family Herpesviridae. This virus is an enveloped, double-stranded DNA virus that primarily causes genital herpes in humans, though it can also infect oral or other mucosal sites. Like other herpesviruses, HSV-2 establishes latent infections in sensory neurons, with potential for periodic reactivation. The species name Humanalpha2 reflects its host specificity (human) and its placement within the Alphaherpesvirinae subfamily, known for rapid replication and neurotropism.

Genital herpes

A common sexually transmitted infection caused primarily by Herpes Simplex Virus type 2 (HSV-2), and sometimes by HSV-1, which more often causes oral herpes. It spreads through skin-to-skin contact, typically during vaginal, anal, or oral sex, even when no visible sores are present. The virus establishes latent infection in nerve cells, leading to recurring outbreaks of painful blisters or ulcers in the genital or anal area. While there’s no cure, antiviral medications can reduce symptoms, frequency of outbreaks, and transmission risk. Genital herpes is biologically stealthy—often asymptomatic, yet persistent and emotionally challenging.

Adsorption (microbiology)

The first step of viral infection where a virus binds to specific receptors on the surface of a host cell, using viral proteins like spikes or tail fibers to recognize and attach to compatible molecules. This interaction is highly selective, determining the virus’s host range and tissue tropism, and is essential for initiating entry into the cell. For bacteriophages, adsorption involves tail structures anchoring to bacterial cell walls, while animal viruses often use glycoproteins to dock with membrane receptors. Adsorption is the molecular handshake that sets the stage for viral invasion.

Penetration (microbiology)

(Microbiology) the stage following adsorption during viral infection, where the virus enters the host cell to deliver its genetic material. This can occur through membrane fusion (common in enveloped viruses), endocytosis (where the cell engulfs the virus), or direct injection of nucleic acid (as seen in bacteriophages). Penetration is a critical transition—from surface attachment to intracellular presence—enabling the virus to hijack the host’s machinery for replication. It’s the molecular breach that turns contact into conquest.

Membrane fusion

A biological process where two separate lipid bilayers—typically the membrane of a virus and the membrane of a host cell—merge into one continuous membrane, allowing the viral contents to enter the cell. This mechanism is common among enveloped viruses, such as herpesviruses and influenza, which use specialized fusion proteins to initiate and mediate the merging. Fusion can occur directly at the cell surface or within endosomes after endocytosis. It’s a molecular gateway—precise, irreversible, and essential for viral entry and infection.

Endocytosis

An energy-dependent process by which eukaryotic cells internalize external substances by engulfing them with their plasma membrane to form vesicles. This mechanism allows cells to import nutrients, fluids, signaling molecules, and even pathogens, and is essential for maintaining cellular homeostasis and communication. There are three main types: phagocytosis, which involves the uptake of large particles; pinocytosis, which absorbs fluids and small solutes; and receptor-mediated endocytosis, which selectively internalizes specific molecules bound to surface receptors. Once inside, the vesicles often fuse with lysosomes for digestion or processing.

Uncoating

The process by which a virus, after entering a host cell, removes its capsid to release its genetic material into the cytoplasm or nucleus. This step is essential for viral replication, as the exposed DNA or RNA can now interact with the host’s machinery. Uncoating mechanisms vary: some viruses disassemble in the cytoplasm, others use host enzymes or pH changes within endosomes. It’s the molecular shedding—where the virus strips down to its core, ready to rewrite the cell’s script.

Anabolism 

(Also known as biosynthesis) the set of metabolic processes in living organisms that build complex molecules from simpler ones, requiring energy input to drive biosynthesis and support growth, repair, and cellular function. These reactions construct macromolecules such as proteins, nucleic acids, lipids, and carbohydrates from smaller units like amino acids and sugars, using energy typically supplied by ATP or NADPH. Anabolism is essential for tissue development, enzyme production, and maintaining structural integrity, and it operates in balance with catabolism, which breaks down molecules to release energy that fuels anabolic activity.

Nucleus (microbiology)

(Microbiology) the membrane-bound organelle found in eukaryotic cells that contains the cell’s genetic material (DNA) organized into chromosomes. It serves as the control center for cellular activities, regulating gene expression, replication, and cell division. The nucleus is enclosed by a nuclear envelope with pores that manage molecular traffic between the nucleus and cytoplasm. Prokaryotic cells, like bacteria, lack a nucleus, instead keeping their DNA in a region called the nucleoid. So in microbiology, the nucleus marks the divide between prokaryotic simplicity and eukaryotic complexity.

Cytoplasm

The gel-like substance inside a cell, enclosed by the cell membrane, where most metabolic activities occur. It contains water, enzymes, nutrients, salts, and various organelles (in eukaryotes) or ribosomes and genetic material (in prokaryotes), all suspended in a semi-fluid matrix called cytosol. The cytoplasm supports cellular structure, facilitates transport, and hosts biochemical reactions essential for growth and survival. It’s the cell’s internal terrain—dynamic, reactive, and vital for life’s choreography.

Maturation and assembly

The final stages of the viral replication cycle where newly synthesized viral components—genomes, capsid proteins, and enzymes—are assembled into complete virions. During assembly, structural proteins form the capsid around the viral nucleic acid, sometimes incorporating enzymes or accessory proteins. Maturation follows, involving structural rearrangements or enzymatic modifications that make the virus infectious. For some viruses, maturation occurs after release from the host cell. Together, these steps transform raw molecular parts into fully formed viral agents—precise, potent, and ready to infect.

Virion

The complete, fully assembled, and infectious form of a virus outside a host cell. It consists of the viral genome (DNA or RNA) enclosed within a protein coat called a capsid, and in some viruses, an additional lipid envelope derived from the host cell membrane. Virions are the transmission units of viruses—stable, compact, and designed to survive in the environment until they encounter a suitable host. They represent the virus at its most mobile and infectious stage, ready to initiate a new cycle of infection.

Release

The final step of the viral replication cycle where newly formed virions exit the host cell to infect new cells. This can occur through cell lysis (breaking open the cell, common in non-enveloped viruses) or budding (where enveloped viruses acquire a lipid envelope from the host membrane as they exit). Release marks the virus’s transition from internal replication to external spread—efficient, often destructive, and essential for propagation.

Naked virus

A virus that lacks a lipid envelope, consisting only of a nucleic acid genome enclosed within a protein capsid. These viruses rely solely on their capsid proteins for attachment and entry into host cells, often using receptor-mediated endocytosis. Naked viruses tend to be more resistant to environmental stressors like heat, drying, and detergents compared to enveloped viruses, making them more stable outside the host. Examples include adenoviruses and picornaviruses. They’re stripped down but resilient—lean infectious machines built for survival.

Enveloped virus

A virus that possesses an outer lipid membrane, called an envelope, surrounding its protein capsid. This envelope is typically derived from the host cell membrane during viral replication or release and is embedded with viral glycoproteins (often called spikes) that facilitate attachment and entry into new host cells. Enveloped viruses—such as HIV, influenza, and herpesviruses—are generally less stable outside the host, being more sensitive to heat, drying, and detergents. The envelope enhances infectivity but also introduces vulnerability, making these viruses biologically stealthy yet fragile.

Virus infection

The process by which a virus enters a host organism, invades its cells, and hijacks cellular machinery to replicate itself. This involves several key steps: adsorption (attachment to the cell), penetration (entry), uncoating (release of viral genome), biosynthesis (production of viral components), assembly (formation of new virions), and release (exit to infect other cells). Virus infection can lead to cell damage, immune responses, and in some cases, disease symptoms. It’s a molecular takeover—stealthy, strategic, and often relentless.

Acute infection

A type of infection that develops rapidly, produces intense symptoms, and typically resolves within a short duration, often days to weeks. It involves a swift onset of pathogen replication, immune response, and clinical signs such as fever, inflammation, or pain. Examples include influenza, mumps, and norovirus infections. Unlike chronic infections, acute infections either clear quickly or lead to host death, making them biologically urgent and often dramatic in presentation.

Mumps

A contagious viral infection caused by the mumps virus, a member of the Paramyxoviridae family, that primarily targets the salivary glands, especially the parotid glands, leading to painful swelling near the jaw and cheeks. It spreads through respiratory droplets, saliva, or contaminated surfaces, and after entering the respiratory tract, the virus replicates and disseminates to various tissues. Symptoms typically include fever, headache, muscle aches, and parotitis, though complications like orchitis, meningitis, or hearing loss can occur. Mumps is preventable through the MMR vaccine, which has significantly reduced its incidence in vaccinated populations.

Influenza

A highly contagious viral infection caused by influenza viruses, primarily types A and B, that target the respiratory tract, leading to symptoms such as fever, cough, sore throat, muscle aches, and fatigue. These viruses belong to the Orthomyxoviridae family and are characterized by their enveloped, single-stranded, negative-sense RNA genomes. Influenza A viruses are further classified by surface proteins—hemagglutinin (HA) and neuraminidase (NA)—into subtypes like H1N1, which contribute to seasonal epidemics and occasional pandemics. The virus spreads through respiratory droplets, and its rapid mutation via antigenic drift and shift makes annual vaccination essential for public health.

Poliomyelitis (polio)

A highly infectious viral disease caused by the poliovirus, which primarily targets the nervous system, potentially leading to paralysis. The virus is transmitted through the fecal-oral route, often via contaminated water or food, and initially replicates in the intestinal tract before spreading to the central nervous system. While many infections are asymptomatic or mild, severe cases can result in muscle weakness, paralysis, or even death. Polio has been largely controlled through widespread vaccination, using either the inactivated polio vaccine (IPV) or the oral polio vaccine (OPV), making eradication a global health goal

Subacute sclerosing panencephalitis

A rare, progressive, and often fatal neurological disorder caused by a persistent, mutated measles virus that remains in the brain after an initial infection, typically occurring in children or adolescents several years post-measles. The virus triggers chronic inflammation, demyelination, and degeneration of brain tissue, leading to symptoms such as personality changes, cognitive decline, motor dysfunction, seizures, and eventually coma and death. SSPE reflects a delayed viral complication and underscores the critical importance of measles vaccination in preventing long-term, devastating outcomes.

Persistent viral infection

One in which a virus remains in the host for an extended period, often months, years, or even a lifetime, rather than being cleared after an acute phase. During this time, the virus may continue to replicate at low levels, remain dormant (latent), or periodically reactivate. Persistent infections can be chronic (ongoing replication, like hepatitis B), latent (dormant with occasional reactivation, like herpesviruses), or slow (gradual progression, like HIV or SSPE). These infections often evade immune detection and can lead to long-term tissue damage or disease.

Reservoir

(Microbiology) any natural habitat—living or non-living—where a pathogen normally lives, grows, and multiplies, serving as a long-term source of infection. Reservoirs can be humans, animals, or environmental sources like soil, water, or surfaces, and they may carry the pathogen symptomatically or asymptomatically. For example, humans are reservoirs for measles, while rodents can be reservoirs for hantavirus. The reservoir sustains the pathogen’s presence in nature, enabling transmission to susceptible hosts and shaping the ecology of infectious disease.

Chronic viral infection

A type of persistent infection in which a virus continues to replicate within the host over an extended period, often months or years, without being fully cleared by the immune system. Unlike latent infections, chronic infections involve ongoing viral production, which may cause gradual tissue damage, immune exhaustion, or inflammation. Examples include hepatitis B, hepatitis C, and HIV. These infections often evade immune defenses through mutation or immune modulation, making treatment complex and long-term.

Hepatitis B

A viral infection caused by the hepatitis B virus (HBV), a partially double-stranded DNA virus from the Hepadnaviridae family, that primarily targets the liver, leading to inflammation, jaundice, and in some cases, chronic liver disease such as cirrhosis or hepatocellular carcinoma. It spreads through blood and bodily fluids, including via sexual contact, needle sharing, or mother-to-child transmission during birth. HBV can cause both acute and chronic infections, with chronic cases persisting for years and often remaining asymptomatic until liver damage occurs. Vaccination is highly effective and forms the cornerstone of global prevention efforts.

Fever

A temporary increase in body temperature, usually in response to infection, inflammation, or illness. It is triggered by pyrogens—substances like interleukin-1 (IL-1) or bacterial endotoxins—that act on the hypothalamus, the brain’s temperature-regulating center. Fever is part of the innate immune response and helps the body fight infection by slowing down pathogen growth and enhancing immune activity.

Nausea

The unpleasant sensation of needing to vomit, often described as a queasy or unsettled feeling in the stomach and throat. It can be triggered by a wide range of factors, including infection, motion, pregnancy, medications, or emotional stress, and may or may not lead to actual vomiting. In microbiology and medicine, nausea is considered a non-specific symptom that signals disruption in the gastrointestinal system or central nervous system, often mediated by the vomiting center in the brainstem and influenced by chemical, neural, or psychological stimuli.

Jaundice

A condition characterized by a yellowing of the skin, eyes, and mucous membranes due to elevated levels of bilirubin in the blood. Bilirubin is a yellow pigment produced during the breakdown of red blood cells, and jaundice occurs when the liver fails to properly process or excrete it, often due to liver disease, bile duct obstruction, or hemolysis. In microbiology and medicine, jaundice is a clinical sign, not a disease itself, and it often signals underlying issues such as hepatitis, cirrhosis, or gallstones.

Latent viral infection

A type of persistent infection in which a virus enters the host and becomes dormant, meaning it remains in cells without actively replicating or causing symptoms for extended periods. During latency, the viral genome is maintained in host cells—often in neurons or immune cells—but no infectious particles are produced. The virus can later reactivate due to stress, immunosuppression, or other triggers, leading to renewed replication and symptoms. Classic examples include herpes simplex virus (HSV) and varicella-zoster virus (VZV), which can cause recurrent cold sores or shingles, respectively.

Provirus

The viral genome that has been integrated into the DNA of a host cell, becoming a permanent part of the host’s genetic material. This typically occurs with retroviruses, like HIV, which use reverse transcriptase to convert their RNA into DNA before insertion. Once integrated, the provirus can remain latent, silently copied during cell division, or become active, producing new viral particles. The provirus stage is crucial for viral persistence, immune evasion, and long-term infection dynamics.

Varicella-zoster virus

(Also known as herpes-zoster virus, when it latently reactivates) A double-stranded DNA virus from the Herpesviridae family that causes chickenpox (varicella) during primary infection and shingles (herpes zoster) upon reactivation. After initial infection—usually in childhood—VZV becomes latent in sensory nerve ganglia, where it can remain dormant for decades. Reactivation, often triggered by aging or immunosuppression, leads to shingles, a painful, localized skin rash. VZV spreads through respiratory droplets or direct contact with lesions and is highly contagious during the chickenpox phase. Vaccines are available for both chickenpox and shingles, making VZV a key example of a latent viral infection with significant public health relevance.

Varicella

(Also commonly known as chicken pox) a highly contagious disease caused by the varicella-zoster virus (VZV), a member of the Herpesviridae family. It typically affects children and presents with a fever, fatigue, and a distinctive itchy rash that progresses from red spots to fluid-filled blisters and then scabs. The virus spreads through respiratory droplets or direct contact with lesions. After recovery, VZV remains latent in nerve ganglia, with the potential to reactivate later in life as shingles (herpes zoster). Vaccination has significantly reduced the incidence and severity of varicella worldwide.

Herpes-zoster

(Also commonly known as shingles) a painful skin rash caused by the reactivation of varicella-zoster virus (VZV), the same virus that causes chickenpox. After a person recovers from chickenpox, VZV remains latent in sensory nerve ganglia and can reactivate years later, typically due to aging, stress, or immunosuppression. The rash usually appears as a band or strip of blisters on one side of the body or face, often accompanied by burning pain, tingling, or itching. Herpes zoster is more common in older adults and can lead to complications like postherpetic neuralgia, a persistent nerve pain after the rash resolves. Vaccination can reduce the risk and severity of shingles.

Tumor

An abnormal mass of tissue that forms when cells grow and divide uncontrollably, bypassing normal regulatory mechanisms. Tumors can be benign (non-cancerous, localized, and slow-growing) or malignant (cancerous, invasive, and capable of spreading to other tissues). They may arise due to genetic mutations, viral infections (like HPV or Epstein-Barr virus), or environmental factors that disrupt cell cycle control. Tumors often alter surrounding tissue architecture and can interfere with organ function depending on their location and behavior.

Benign tumor

A non-cancerous mass of cells that grows in a localized and controlled manner, without invading nearby tissues or spreading to other parts of the body. These tumors arise from abnormal but non-aggressive cell proliferation, often due to genetic or environmental factors, and typically maintain normal tissue architecture. While benign tumors can cause symptoms by compressing surrounding structures, they usually grow slowly and are often encapsulated, making them easier to remove surgically. Unlike malignant tumors, benign ones do not metastasize and generally pose less risk to overall health.

Malignant tumor

A cancerous mass of cells characterized by uncontrolled growth, invasion of nearby tissues, and the potential to spread (metastasize) to distant parts of the body through the blood or lymphatic system. These tumors arise from genetic mutations that disrupt normal cell cycle regulation, allowing cells to proliferate aggressively and evade immune detection. Malignant tumors often alter tissue architecture, compromise organ function, and require complex treatment strategies such as surgery, chemotherapy, or radiation. Unlike benign tumors, malignant ones pose a significant threat to health due to their invasive and metastatic behavior.

Proto-oncogene

A normal gene that plays a key role in cell growth, division, and survival, but can become an oncogene if mutated or abnormally expressed. In their healthy state, proto-oncogenes encode proteins like growth factors, receptors, or signal transducers that regulate the cell cycle and ensure controlled proliferation. However, when altered—through point mutations, gene amplification, or chromosomal translocations—they can drive uncontrolled cell division, contributing to cancer development. Proto-oncogenes are essential for normal cellular function, but their dysregulation is a critical step in tumorigenesis.

Oncogene

A mutated or overactive version of a proto-oncogene that drives uncontrolled cell growth and division, contributing to the development of cancer. Normally, proto-oncogenes regulate essential processes like cell cycle progression, growth signaling, and survival, but when altered—through point mutations, gene amplification, or chromosomal translocations—they become oncogenes that bypass regulatory checkpoints and promote tumor formation. Oncogenes can encode hyperactive proteins or cause excessive expression, tipping the balance toward malignant transformation. They are central players in the molecular biology of cancer.

Tumor suppressor gene

A type of gene that encodes proteins responsible for regulating cell growth, repairing DNA damage, and promoting programmed cell death (apoptosis) to prevent uncontrolled cell division. When functioning properly, these genes act as cellular brakes, ensuring that cells do not proliferate excessively or survive when damaged. However, if a tumor suppressor gene is mutated, deleted, or silenced, its protective functions are lost, allowing cells to grow unchecked and potentially form malignant tumors. Key examples include TP53, RB1, and BRCA1/2, which are often inactivated in various cancers.

Oncogenic virus

A virus capable of triggering cancer development by disrupting normal cellular regulation, often through insertion of viral genes, activation of oncogenes, or inactivation of tumor suppressor genes. These viruses may carry DNA or RNA genomes and establish persistent or latent infections that interfere with the host cell cycle, leading to uncontrolled proliferation. Notable examples include human papillomavirus (HPV), Epstein-Barr virus (EBV), hepatitis B and C viruses (HBV, HCV), and Kaposi sarcoma-associated herpesvirus (KSHV). Oncogenic viruses contribute to approximately 15–20% of human cancers worldwide, especially in regions with high viral prevalence.

DNA virus

A type of virus that contains DNA as its genetic material, either in double-stranded (dsDNA) or single-stranded (ssDNA) form, and typically replicates within the host cell nucleus using the host’s or its own DNA polymerase. These viruses are classified in the Baltimore system as Class I (dsDNA) or Class II (ssDNA), and include families like Herpesviridae, Adenoviridae, and Papillomaviridae. DNA viruses often have stable genomes, enabling long-term latency, and some—such as HPV or Epstein-Barr virus—can contribute to oncogenesis by disrupting host cell cycle regulation.

RNA virus

A virus that uses ribonucleic acid (RNA) as its genetic material, either in single-stranded (ssRNA) or double-stranded (dsRNA) form, and typically replicates in the cytoplasm of host cells using RNA-dependent enzymes. These viruses are classified in the Baltimore system as Groups III (dsRNA), IV (+ssRNA), V (−ssRNA), and VI (retroviruses, which use reverse transcriptase). Due to the lack of proofreading during replication, RNA viruses have high mutation rates, enabling rapid evolution and adaptation. Notable RNA viruses include influenza virus, SARS-CoV-2, HIV, and measles virus, many of which are responsible for major human diseases.

Hepatitis C

A liver disease caused by the hepatitis C virus (HCV), a single-stranded RNA virus from the Flaviviridae family that spreads primarily through blood-to-blood contact, such as sharing needles or receiving unscreened transfusions. Once inside the body, HCV targets hepatocytes, leading to inflammation that can progress to chronic hepatitis, cirrhosis, or liver cancer if untreated. The infection is often asymptomatic, making early detection difficult, and while there is no vaccine, modern direct-acting antivirals (DAAs) can cure most cases. HCV’s high mutation rate and ability to establish persistent infection make it a major global health concern.

Epstein-Barr virus

A double-stranded DNA virus from the Herpesviridae family that primarily infects B lymphocytes and epithelial cells, establishing lifelong latency in the host. It spreads mainly through saliva and is the causative agent of infectious mononucleosis, especially in adolescents and young adults. EBV is also linked to several cancers, including Burkitt lymphoma, nasopharyngeal carcinoma, and Hodgkin lymphoma, making it the first identified human oncogenic virus. Its ability to manipulate host immune responses and persist in latent form contributes to its role in chronic disease and oncogenesis.

Mononucleosis

(Also known as “mono” or “the kissing disease”) a viral illness most commonly caused by Epstein-Barr virus (EBV), a member of the Herpesviridae family. It typically affects adolescents and young adults, spreading through saliva and close personal contact. Symptoms include fever, sore throat, fatigue, swollen lymph nodes, and sometimes enlarged spleen or liver. The infection targets B lymphocytes, leading to an immune response marked by atypical lymphocytes in the blood. While usually self-limiting, recovery can take weeks, and rest is the primary treatment.

Human papillomavirus (HPV)

A group of small, double-stranded DNA viruses from the Papillomaviridae family that infect epithelial cells of the skin and mucous membranes, primarily transmitted through sexual contact. There are over 100 types, with low-risk strains causing warts and high-risk strains—like HPV-16 and HPV-18—linked to cervical, anal, and oropharyngeal cancers. HPV disrupts normal cell cycle regulation by interfering with tumor suppressor proteins, such as p53 and Rb, contributing to oncogenesis. While many infections are asymptomatic and cleared naturally, persistent infection with high-risk types poses serious health risks. Vaccines like Gardasil and Cervarix offer effective protection against the most dangerous strains.

Cervical cancer

A malignant disease that originates in the cells lining the cervix, the lower part of the uterus that connects to the vagina, and is most commonly caused by persistent infection with high-risk human papillomavirus (HPV) types, especially HPV-16 and HPV-18. The disease often begins as precancerous changes (dysplasia) that can progress to invasive cancer if not detected and treated early. Symptoms may include abnormal vaginal bleeding, pelvic pain, or pain during intercourse, though early stages are frequently asymptomatic. Regular screening with Pap smears and HPV testing, along with vaccination, are key strategies for prevention and early detection. When caught early, cervical cancer is highly treatable, making public health efforts critical in reducing its global burden.

Viroid

A small, circular, single-stranded RNA molecule that infects plants, causing disease without encoding any proteins. Unlike viruses, viroids lack a protein coat (capsid) and do not produce viral proteins; instead, they interfere with host gene expression by mimicking or disrupting regulatory RNA. Viroids replicate using the host’s RNA polymerase and are transmitted through mechanical damage, contaminated tools, or vegetative propagation. They are among the smallest known infectious agents, with examples like potato spindle tuber viroid (PSTVd) causing significant agricultural losses. Viroids are classified into families such as Pospiviroidae and Avsunviroidae.

Prion

An infectious, misfolded form of a normal cellular protein—typically found in the brain—that can induce other normal proteins to also misfold into the abnormal shape. Unlike viruses or bacteria, prions contain no nucleic acids (DNA or RNA) and propagate by converting properly folded versions of the same protein into the disease-causing form. This accumulation of misfolded proteins leads to neurodegenerative diseases known as transmissible spongiform encephalopathies (TSEs), such as Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy (mad cow disease) in cattle.

Transmissible spongiform encephalopathies

A group of rare, fatal neurodegenerative diseases caused by prions—abnormally folded proteins that induce misfolding in normal brain proteins. These diseases affect both humans and animals, leading to progressive brain damage characterized by spongiform changes, or sponge-like holes in neural tissue. TSEs include conditions such as Creutzfeldt-Jakob disease (CJD) in humans, bovine spongiform encephalopathy (BSE) in cattle, and scrapie in sheep. Transmission can occur through infected neural tissue, contaminated surgical instruments, or consumption of infected meat, and there is currently no cure or vaccine, making them a major concern in public health and food safety.

Scrapie

A transmissible spongiform encephalopathy (TSE) that affects sheep and goats, caused by prions—misfolded proteins that induce abnormal folding in normal brain proteins. It leads to progressive neurodegeneration, with symptoms including behavioral changes, lack of coordination, intense itching, and wasting. The name “scrapie” comes from the affected animals’ tendency to rub or scrape their skin due to pruritus. Prions accumulate in the central nervous system, creating sponge-like holes in brain tissue. Scrapie is contagious within flocks, transmitted through contact with infected placental tissue or fluids, and has no cure, making it a major concern in veterinary and agricultural health.

Mad cow disease

A fatal neurodegenerative disease that affects cattle and is caused by prions—abnormally folded proteins that induce misfolding in normal brain proteins. It leads to progressive brain damage, with symptoms such as aggression, lack of coordination, and weight loss, eventually resulting in death. The disease causes spongiform changes in brain tissue, giving it a sponge-like appearance under the microscope. BSE can be transmitted to humans through consumption of contaminated beef, leading to variant Creutzfeldt-Jakob disease (vCJD). There is no cure, and strict regulations on animal feed and surveillance have been key to controlling its spread.

Creutzfeldt-Jakob disease

A rare, fatal neurodegenerative disorder caused by prions—misfolded proteins that trigger abnormal folding in normal brain proteins, leading to rapid brain deterioration. It typically affects older adults, with symptoms such as memory loss, behavioral changes, muscle stiffness, and coordination problems, progressing quickly to coma and death. CJD can occur sporadically, be inherited, or result from contaminated medical procedures or consumption of infected meat (as in variant CJD). The disease causes spongiform changes in brain tissue and has no cure, making early diagnosis and strict infection control essential.