Virus basics

• Viruses are obligate intracellular parasites, meaning they are infectious, acellular agents that lack the cellular machinery for self-replication and thus cannot reproduce or carry out metabolic processes outside a living host cell.

• Virus particles (virions) are primarily composed of a packaged nucleic acid genome (either DNA or RNA, but never both) and a protective protein shell called a capsid; viral genomes encode a limited set of regulatory proteins (e.g., RNA replicases, reverse transcriptases) that are crucial for hijacking the host cell's existing transcriptional and translational machinery to produce new viral components.

Virus diversity and sizes

• Viruses exhibit vast diversity in their genome type, which dictates their replication strategy according to the Baltimore classification system: negative-sense ssRNA, positive-sense ssRNA, double-stranded DNA (dsDNA), RNA retroviruses (integrating their RNA after reverse transcription into DNA), double-stranded RNA (dsRNA), and single-stranded DNA (ssDNA).

• Examples:

  • Influenza A: features a segmented negative-sense ssRNA genome; viral particle size is typically around 80-120\ \mathrm{nm} in diameter, but can vary.

  • Coronaviruses (e.g., SARS-CoV-2): possess a positive-sense ssRNA genome, which can directly serve as mRNA for protein synthesis; virion size is approximately 80-120\ \mathrm{nm} with distinctive club-shaped spike glycoproteins protruding from the surface.

  • HIV (retrovirus): contains two identical copies of a positive-sense RNA genome and carries its own reverse transcriptase enzyme to convert RNA into DNA inside the host cell. The virion is roughly 100-120\ \mathrm{nm} in diameter.

  • Herpesviridae: large dsDNA viruses, typically 150-200\ \mathrm{nm} or larger, characterized by an icosahedral capsid, a tegument layer, and an outer lipid envelope derived from the host membrane.

  • Bacteriophage T4: a complex dsDNA virus, significantly larger than many animal viruses, measuring about 200\ \mathrm{nm} long (including tail fibers) with an icosahedral head of
    approximately 85\ \mathrm{nm} diameter; it infects bacteria and has a distinct head-and-tail morphology.

  • Mimivirus (Acanthamoeba polyphaga mimivirus, APMV): an extraordinarily large dsDNA virus with a complex genome and a particle size of about 400-800\ \mathrm{nm}, blurring the lines between viruses and cellular organisms.

• Virus sizes vary dramatically, spanning from very small (tens of nanometers) to giant viruses affecting eukaryotic cells (nearly a micron-scale); examples include Rhinovirus (a picornavirus,
  around 20-30\ \mathrm{nm}), Tobacco mosaic virus (a rod-shaped virus,
  300\ \times\ 18\ \mathrm{nm}), Ebola virus (a filamentous virus, up to
  14,000\ \mathrm{nm} long but typically
  970\ \mathrm{nm}} average in length and 80\ \mathrm{nm}} in diameter), and vaccinia virus (a poxvirus,
  a complex brick-shape of
  360\ \times\ 270\ \times\ 250\ \mathrm{nm}}).

Virus structure and components

• Core components:

  • Nucleic acid: The genetic material, either DNA (single-stranded or double-stranded) or RNA (single-stranded, double-stranded, positive-sense, or negative-sense), encoding the essential viral genes.

  • Capsid: A protein shell that encloses and protects the viral genome. It is constructed from repeating protein subunits called capsomeres, which self-assemble into various symmetrical shapes (e.g., icosahedral, helical).

• Optional structures:

  • Envelope with surface spikes: A lipid bilayer derived from the host cell membrane (e.g., plasma membrane, nuclear membrane) that surrounds the capsid, often embedded with viral glycoproteins (spikes) important for host cell attachment and entry.

  • Complex components: Structures found in some viruses that go beyond simple helical or icosahedral capsids, such as the tail fibers, baseplate, and sheath of bacteriophages, or the lateral bodies of poxviruses.

• Enveloped viruses acquire their lipid envelope during maturation when newly formed virions bud through a host cell membrane, picking up a portion of the membrane along with viral glycoproteins inserted into it.

Morphology and notable virus types

• Bacteriophage: These viruses exhibit a complex morphology, typically consisting of an icosahedral head (capsid) that encapsulates the DNA, a rigid hollow protein tail connected to a baseplate, and multiple tail fibers. The tail fibers facilitate specific attachment to bacterial cell surface receptors, and the tail then contracts to inject the viral DNA into the bacterial cytoplasm, leaving the capsid outside.

• Enveloped vs. non-enveloped: Enveloped viruses possess a lipid bilayer derived from the host cell membrane, containing viral glycoproteins (spikes) that are critical for specific binding to host cell receptors and facilitating entry via membrane fusion or endocytosis. Non-enveloped (naked) viruses lack this outer lipid layer and rely solely on direct interactions between their capsid proteins and host cell receptors for attachment and entry, often through pore formation or cell lysis for release.

Viral taxonomy and dimensions (high-level)

• Classification criteria for viruses are multifaceted and include:

  • Nucleic acid type: (DNA or RNA, single-stranded vs. double-stranded, linear vs. circular, segmented vs. non-segmented).

  • Host type: (animal, plant, bacterial, fungal).

  • Genome organization: (e.g., presence of reverse transcriptase, site of replication).

  • Morphology: (e.g., icosahedral, helical, complex, presence or absence of an envelope).

• Representative families (sizes are approximate and can vary):

  • Parvoviridae: Smallest DNA viruses; ssDNA, non-enveloped, icosahedral capsid; approximately 18-25\ \mathrm{nm} in diameter. They require actively dividing host cells for replication.

  • Adenoviridae: Medium-sized dsDNA, non-enveloped, icosahedral capsid with distinctive fibers (spikes) at the vertices; generally 70-90\ \mathrm{nm} in diameter. Known for causing respiratory infections.

  • Poxviridae: Very large, complex dsDNA viruses, enveloped, with an unusual brick-shaped morphology; ranging from 200-450\ \mathrm{nm} in length. They replicate entirely in the cytoplasm.

  • Herpesviridae: Large dsDNA viruses, enveloped, icosahedral capsid; around 150-200\ \mathrm{nm} in diameter. Characterized by latency after primary infection.

  • Papillomaviridae (HPV) and Polyomaviridae: Small dsDNA viruses, non-enveloped, icosahedral; approximately 40-55\ \mathrm{nm} in diameter. HPV causes warts and certain types are strongly associated with a high risk of developing various cancers (e.g., cervical, anal, oral).

General life cycle of a virus

• Attachment (adsorption): The virion specifically binds to receptor molecules on the surface of a susceptible host cell. This interaction is mediated by viral surface proteins (spikes for enveloped, capsid proteins for non-enveloped) and host cell surface receptors.

• Penetration and uncoating: The virus enters the host cell through various mechanisms (e.g., direct penetration, membrane fusion, endocytosis). Once inside, the viral capsid is degraded, releasing the viral nucleic acid genome into the host cell cytoplasm or nucleus.

• Synthesis (replication and gene expression): The viral genome is replicated using host cell machinery (and sometimes viral enzymes). Viral genes are transcribed into mRNA and then translated into viral proteins, including structural proteins for new virions and non-structural proteins (e.g., enzymes, regulatory proteins) to facilitate replication and manipulate host cell functions.

• Assembly (maturation): Newly synthesized viral nucleic acids and proteins self-assemble into new, immature virions (nucleocapsids) within the host cell.

• Release: Mature virions exit the host cell. Enveloped virions typically bud from the cell, acquiring their envelope from the host membrane (e.g., plasma membrane, ER). Non-enveloped viruses often cause lysis (bursting) of the host cell to release progeny phages.

Adsorption and entry mechanisms

• Specific receptor-spike/capsid interactions drive attachment: Viral surface proteins (glycoproteins for enveloped viruses like influenza, or specific capsid proteins for naked viruses like poliovirus) bind with high specificity to complementary receptor molecules (e.g., proteins, carbohydrates, lipids) on the host cell surface. This binding determines the tropism (which cell types or species a virus can infect).

• Entry pathways:

  • Endocytosis with vesicle/internalization: Many viruses enter by endocytosis, where the host cell engulfs the virion in a vesicle (e.g., clathrin-mediated endocytosis for influenza, caveolae-mediated endocytosis for SV40). Acidification of the endosome often triggers conformational changes in viral proteins, leading to uncoating or membrane fusion.

  • Membrane fusion (for enveloped viruses): Enveloped viruses (e.g., HIV, measles, herpesviruses) can directly fuse their viral envelope with the host cell plasma membrane, releasing the nucleocapsid directly into the cytoplasm. This process is mediated by specific viral fusion proteins.

• Uncoating follows entry to release the viral genome from its protective capsid, making it accessible for replication and gene expression by host (or viral) enzymes.

Viral families to know (examples)

• Major human pathogens include: Influenza (orthomyxovirus, segmented RNA), HIV (retrovirus, RNA), SARS-CoV-2 (coronavirus, large RNA), Measles (paramyxovirus, RNA), Polio (picornavirus, non-enveloped RNA), Rabies (rhabdovirus, RNA), Ebola (filovirus, RNA), Zika (flavivirus, RNA), HPV (papillomavirus, DNA), HBV (hepadnavirus, DNA), HCV (flavivirus, RNA), Dengue (flavivirus, RNA), West Nile (flavivirus, RNA), Smallpox (poxvirus, DNA), Adenovirus (DNA), Rhinovirus (picornavirus, RNA), Norovirus (calicivirus, RNA), Rotavirus (reovirus, segmented dsRNA), Tobacco mosaic virus (plant virus, RNA), Bacteriophages (diverse viruses infecting bacteria).

• Practical takeaway: Understanding the diverse genome types, presence/absence of envelopes, and fundamental replication strategies allows for rapid comparison of viruses, prediction of their behavior, and development of targeted antiviral therapies.

Lytic vs. lysogenic cycles

• Lytic cycle: This is a virulent infection pathway characterized by rapid hijacking of host cellular machinery for massive production of new virions, culminating in the lysis (destruction) of the host cell to release the progeny viruses. Examples include many bacteriophages (e.g., T4) and human viruses like influenza and norovirus. The cycle is typically completed within a short timeframe (e.g., minutes for phages, hours for animal viruses).

• Lysogenic cycle: This alternative pathway, primarily observed in bacteriophages (temperate phages), involves the integration of the viral DNA into the host bacterial chromosome, forming a prophage. The prophage DNA is then replicated along with the host genome during cell division without causing immediate harm to the host. Under specific environmental stresses (e.g., UV radiation, chemical mutagens), the prophage can be induced to excise from the host genome and enter the lytic cycle.

• Bacteriophages can utilize either cycle; lysogeny aids viral survival by ensuring genome propagation when host cells are scarce or stressed, preserving the viral lineage until conditions are favorable for lytic replication.

The Lytic cycle (conceptual steps)

• Early in infection, the viral genome directs the transcription of early genes, often utilizing the host cell's RNA polymerase. These early genes encode proteins that typically take over host cell functions, such as shutting down host gene expression and preparing the cell for viral DNA replication.

• Early proteins then promote viral genome replication. For example, some early proteins might degrade host DNA, while others synthesize dNTPs or components of a viral DNA polymerase.

• Subsequently, late genes are expressed, which primarily encode structural proteins that will form the new virion particles (e.g., capsomeres, tail fibers). These proteins are synthesized in large quantities.

• Assembly (packaging) of capsids around new viral genomes occurs spontaneously or with the aid of scaffolding proteins. New virions are produced and packaged efficiently within the host cell.

• Finally, genes encoding lytic enzymes (e.g., lysozyme in phages) are expressed, causing the host cell membrane and/or cell wall to degrade, leading to cell lysis and the explosive release of hundreds to thousands of progeny phages.

Lysogenic cycle details

• Phage DNA integrates into a specific site on the host bacterial chromosome, becoming a prophage. This integration is mediated by a viral integrase enzyme.

• The prophage DNA replicates passively with the host DNA during every cell division, effectively transmitting the viral genome to daughter cells without producing new virions. The prophage remains repressed by repressor proteins. Under certain conditions (e.g., DNA damage, nutrient limitation), the prophage excises from the host genome (mediated by an excisionase enzyme) and enters the lytic cycle, leading to virion production and host cell lysis.

Retroviruses and HIV replication

• Retroviruses are a unique class of RNA viruses (e.g., HIV) that carry their own enzyme, reverse transcriptase, and a positive-sense single-stranded RNA genome (typically two copies).

• After entry into the host cell's cytoplasm, the reverse transcriptase converts the viral RNA genome into a complementary DNA (cDNA) strand, and then synthesizes a second DNA strand, resulting in a double-stranded viral DNA copy.

• A viral enzyme called integrase then mediates the insertion of this newly synthesized viral DNA into the host cell's chromosome, where it is referred to as a provirus. This integration is a permanent alteration of the host genome.

• The host cell's own transcriptional machinery recognizes the proviral DNA as part of its genome and transcribes viral mRNA. This mRNA is then translated by host ribosomes to express various viral RNAs (including new RNA genomes) and proteins (e.g., structural proteins, enzymes).

• New virions then assemble at the host cell membrane. Assembled virions bud from the host cell, acquiring their envelope during this process. A viral protease enzyme then cleaves large precursor proteins into smaller functional proteins, a crucial step for the maturation into infectious virions.

HIV life cycle at a glance

• Attachment and entry: HIV's gp120 glycoprotein binds to CD4 receptors and a co-receptor (e.g., CCR5 or CXCR4) on host cells (T helper cells, macrophages). This triggers gp41 to mediate fusion of the viral envelope with the host cell membrane, releasing the capsid into the cytoplasm.

• Reverse transcription: Within the cytoplasm, reverse transcriptase converts the single-stranded RNA genome into a double-stranded proviral DNA.

• Integration: The proviral DNA is transported to the nucleus and inserted into the host cell's chromosome by integrase.

• Transcription/translation: The integrated proviral DNA is transcribed by host RNA polymerase into viral mRNA, which is translated into viral proteins by host ribosomes. This mRNA also serves as the genome for new virions.

• Assembly, budding, and maturation: New viral RNA genomes and proteins assemble at the cell membrane. Immature virions bud off, acquiring an envelope. Viral protease then cleaves precursor proteins within the budding virion, leading to the final maturation into infectious HIV particles.

Anti-HIV drug classes (concepts)

• Inhibitors of virus binding to host cells: These drugs block the interaction between viral surface proteins (e.g., gp120) and host cell receptors (e.g., CD4, CCR5), preventing the virus from attaching and entering the cell (e.g., Entry inhibitors like Maraviroc targeting CCR5).

• Inhibitors of viral internalization: These drugs prevent the fusion of the viral and host membranes, blocking the entry of the viral core into the cytoplasm (e.g., Fusion inhibitors like Enfuvirtide).

• Reverse transcriptase inhibitors (e.g., Azidothymidine (AZT) as a historic example, Tenofovir, Emtricitabine): These drugs block the activity of reverse transcriptase, preventing the conversion of viral RNA into DNA. This class includes nucleoside reverse transcriptase inhibitors (NRTIs), which are DNA chain terminators, and non-nucleoside reverse transcriptase inhibitors (NNRTIs), which directly bind to and inhibit the enzyme.

• Integrase inhibitors (e.g., Raltegravir, Dolutegravir): These drugs block the viral integrase enzyme, which is critical for inserting the viral DNA into the host cell's genome, thereby preventing provirus formation.

• Protease inhibitors (e.g., Saquinavir, Ritonavir): These drugs target the viral protease enzyme, which is responsible for cleaving large viral precursor proteins into smaller, functional proteins required for the assembly and maturation of infectious virions. By inhibiting this step, only immature, non-infectious virus particles are produced.

Cultivation and detection of viruses

• Bacteriophages are typically grown on bacterial lawns using a plaque assay. A dilute mixture of phage and bacteria is mixed with molten agar and poured over a solidified base agar. Each phage infects a bacterium, replicates, lyses it, and infects surrounding bacteria, creating a clear zone (plaque) on the bacterial lawn.

• Animal and plant viruses (which cannot be grown in inanimate media) are cultivated using living host systems:

  • Embryonated eggs: Historically, many animal viruses (e.g., influenza, mumps, measles) are grown by inoculation into specific membranes or cavities of incubated chicken eggs (e.g., allantoic cavity, amniotic cavity).

  • Cell culture (tissue culture): A common and versatile method for growing animal, plant, and some bacteriophages. Cells are grown in vitro in a nutrient medium. Primary cell cultures are derived directly from tissues and have a limited lifespan. Diploid cell strains can be maintained for numerous generations.

• Continuous cell lines (e.g., HeLa cells, Vero cells) are immortalized cells that can be maintained indefinitely in culture, providing a consistent and convenient substrate for virus propagation and research.

Cytopathic effects (CPE)

• Infected cells, particularly in cell culture, may display characteristic morphological or functional changes known as cytopathic effects (CPE) when viewed under a microscope. These changes are crucial for the qualitative detection and identification of viruses in culture and can include:

  • Cell lysis: complete destruction of the cell.

  • Cell rounding and detachment from the culture surface.

  • Syncytia formation: fusion of multiple infected cells into a large, multinucleated giant cell (e.g., measles, herpesviruses).

  • Inclusion bodies: abnormal structures within the cell cytoplasm or nucleus, often sites of viral replication or assembly (e.g., Negri bodies in rabies virus infection).

  • Chromosomal aberrations or altered growth patterns.

Viruses and cancer: oncogenes and tumor suppressors

• Proto-oncogenes are normal cellular genes that regulate cell growth and differentiation. When mutated or overexpressed, they can become oncogenes, which are genes that promote uncontrolled cell proliferation and contribute to cancer development.

• Oncogene activation, often through mutation (e.g., RAS mutations) or overexpression of growth-promoting proteins (e.g., HER2 amplification), drives excessive, unregulated cell growth and division.

• Tumor suppressor genes (e.g., RB, p53) normally act to restrain cell growth, repair DNA damage, or induce apoptosis when necessary, thus preventing uncontrolled cell division. Loss of function (e.g., mutation, deletion) of both copies of a tumor suppressor gene removes these crucial cell cycle brakes and can lead to cancer.

Molecular changes in cancer cells (concepts)

• Overexpression or mutation of growth factor receptors (e.g., HER2 in breast cancer) leads to constitutive signaling pathways that promote cell proliferation even in the absence of external growth factors.

• Inactivation of tumor suppressors (e.g., Retinoblastoma protein (RB)) removes critical checkpoints within the cell cycle. RB normally binds to and inhibits E2F transcription factors, preventing progression from G1 to S phase. When RB is inactivated, E2F is constitutively active, driving cells into division.

• Viral proteins can directly disrupt normal cell cycle controls to promote transformation. For instance, high-risk HPV types integrate their DNA into the host genome, leading to the overexpression of viral oncogenes E6 and E7. HPV E7 protein binds to and inactivates the tumor suppressor protein RB, while HPV E6 binds to and promotes the degradation of the p53 tumor suppressor protein. The combined inactivation of RB and p53 removes key cellular brakes on division, promoting unchecked proliferation and genomic instability, which are hallmarks of cancer.

HPV and cervical cancer progression

• Human Papillomavirus (HPV) infects basal epithelial cells in squamous mucosa, particularly in the cervix. The viral DNA can either exist as an episome or, in rarer cases, integrate into the host cell genome.

• While many HPV infections are transient, with approximately 90\% healing spontaneously within weeks to months, a small fraction (
  around 0.8\% of high-risk HPV infections) can persist and, over years, progress to cervical intraepithelial neoplasia (CIN) and eventually invasive cervical cancer.

• Persistent infection, especially with high-risk HPV types (e.g., HPV16, HPV18), and particularly viral DNA integration into the host genome (which often leads to increased and sustained expression of viral oncogenes E6 and E7), profoundly contributes to malignant transformation by disrupting cell cycle control (inactivating RB and p53) and promoting genomic instability.

HPV transmission and prevention

• Transmission: HPV is primarily transmitted through sexual activity (vaginal, anal, oral sex), but also via direct skin-to-skin contact in genital regions, shared personal items (though less common), and rarely from mother to infant during birth.

• Prevention:

  • Vaccination (HPV vaccine): Highly effective (nearly 100\% efficacy) in preventing infection with the HPV types covered by the vaccine, including those that cause most cervical cancers, anal cancers, and genital warts. It is most effective when administered before sexual activity.

  • Safe sex practices: Consistent and correct use of condoms can reduce, but not entirely eliminate, the risk of HPV transmission since the virus can infect areas not covered by a condom.

  • Limiting the number of sexual partners: Reduces exposure risk.

  • Regular health checkups: Pap tests (Papanicolaou smears) and HPV DNA tests allow for early detection and treatment of precancerous lesions, preventing progression to cancer.

Plant viruses and agricultural impact

• Plant viruses are numerous and cause significant agricultural and economic losses worldwide. For example, Pepper mild mottle virus (PMMoV) is a highly stable tobamovirus that can cause mottling, stunting, and leaf distortion in pepper plants, reducing crop yield and quality.

• Mosaic viruses (e.g., Tobacco mosaic virus, Cucumber mosaic virus) are a broad group of plant viruses that commonly cause characteristic mosaic patterns (alternating light and dark green areas) on leaves, leaf curling, necrosis, and stunting, leading to substantial damage to various crop plants (e.g., tobacco, tomatoes, cucumbers, squash).

Viroids

• Viroids are distinct from viruses; they are the smallest known infectious pathogens. They consist of naked, covalently closed circular single-stranded RNA (ssRNA) genomes, typically ranging from
  200-400\ \mathrm{bases} in length, lacking a protein coat.

• They are exclusively plant pathogens and do not encode any proteins. Instead, they replicate autonomously in the host cell nucleus or chloroplasts, hijacking host RNA polymerase machinery (e.g., DNA-dependent RNA polymerase II) for replication. Pathogenicity is mediated by direct interaction of the viroid RNA with host cell machinery; they can bind to and interfere with plant mRNAs and proteins (e.g., acting as siRNAs) to disrupt normal plant development and cause disease.

Prions and prion diseases

• Prions (Proteinaceous infectious particles) are unique infectious agents comprised solely of misfolded proteins (PrP^{Sc}, prion scrapie form) that cause a group of fatal neurodegenerative diseases known as transmissible spongiform encephalopathies (TSEs). These misfolded proteins template the misfolding of normal cellular prion proteins (PrP^C, cellular prion form) in the brain, leading to their aggregation and the characteristic spongiform brain damage (vacuolization).

• Prions are extraordinarily resistant to conventional sterilization methods, including high heat (even boiling), strong disinfectants, and radiation. This extreme resistance presented major challenges regarding containment and public health during outbreaks, such as Bovine Spongiform Encephalopathy (BSE, or "mad cow disease"), which led to the mass culling and incineration of infected livestock to prevent further spread.

• Associated human diseases include variant Creutzfeldt–Jakob disease (vCJD), which is strongly linked to the consumption of beef products contaminated with prions from BSE-infected cattle, as well as sporadic CJD, familial CJD, Gerstmann–Sträussler–Scheinker syndrome (GSS), and fatal familial insomnia (FFI).

Prion mechanism and conformations

• The normal cellular prion protein (PrP^C) is a glycoprotein found abundantly on the surface of neurons and other cells, commonly containing a high proportion of alpha-helical structures and believed to play roles in cell signaling, adhesion, and neuroprotection.

• The pathogenic prion form (PrP^{Sc}) is a conformational isomer of PrP^C. Unlike PrP^C, PrP^{Sc} has a high beta-sheet content, making it highly stable, insoluble, and protease-resistant. This misfolded PrP^{Sc} acts as a template, inducing a conformational change in nearby normal PrP^C molecules, converting them into the PrP^{Sc} form.

• Prion propagation involves this autocatalytic conformational conversion, leading to the exponential accumulation of PrP^{Sc}. These misfolded proteins aggregate into amyloid-like fibrils or plaques in the brain, disrupting neuronal function and causing irreversible neurodegeneration and brain damage.

Quick takeaways for quick recall

• Viruses are acellular, obligate intracellular parasites that rely entirely on host cells for replication; they exhibit diverse genome types and structures that dictate their host range and pathogenesis.

• Viral life cycles universally include attachment, entry, replication, assembly, and release; lysogeny (integration into host genome) and lysis (cell destruction) offer distinct alternative survival and propagation strategies, particularly for bacteriophages.

• Retroviruses, such as HIV, uniquely integrate their reverse-transcribed DNA genome into the host's chromosome, establishing a permanent provirus; reverse transcription is their enzymatic hallmark.

• Antiretroviral therapy (ART) targets multiple stages of the retroviral life cycle, including host cell entry, reverse transcription, integrase activity, and protease-mediated protein processing, to effectively control viral replication.

• Cultivation methods differ depending on the host: bacteriophages are grown on bacterial lawns (e.g., plaque assay); animal and plant viruses typically require living systems like embryonated eggs or cell culture (e.g., continuous cell lines).

• Some specific viruses contribute to cancer development (oncogenesis) by altering host oncogenes or tumor suppressor genes (e.g., HPV integrating its genome and expressing E6/E7 to inactivate p53/RB, respectively).

• Beyond viruses, distinct infectious agents include plant pathogens like viroids (naked ssRNA) and prions (infectious misfolded proteins that cause neurodegeneration) which require different containment and therapeutic strategies.