Biological and Physical Properties of Viruses

What is the Latin meaning of the word "virus"?

Poison, venom"

How were viruses first described?

As "filterable agents" that passed through filters that retained bacteria.

What type of parasites are viruses?

Obligate intracellular parasites.

Why are viruses considered obligate intracellular parasites?

They depend on the biochemical machinery of the host cell for replication.

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Are viruses visible under a light microscope?

The largest viruses are barely visible in a light microscope. In nanometers ,From 18 nm (parvoviruses) to 300 nm (poxviruses).

: What body systems and fluids are part of the human virome?

Lung, gastrointestinal (GI) tract, reproductive tract, blood, urine, and semen.

What evidence shows that we carry viral genomes as part of our own genetic material?

The presence of LTR retrotransposons and protein-coding genes in our genome.

What are the basic structural characteristics of a virus?

A virus may have RNA or DNA and can be either a naked capsid or an envelope.

How do viruses replicate?

Viral components are assembled; they do not replicate by division.

What are viruses that infect bacteria called?

Bacteriophages (phages).

What is a virus?

An obligate intracellular “microorganism” containing a DNA or RNA genome, a protein coat, and sometimes a lipoprotein envelope.

How did Jean and Peter Medawar describe a virus?

Simply a piece of bad news wrapped in protein.”

What is the fundamental difference between viruses and other infectious agents?

Their mechanism of reproduction—viruses cannot make energy, substrates, proteins, or replicate their genome independently of the host cell.

Why must viruses be adapted to the biochemical rules of the cell?

To use the cell’s biosynthetic machinery for replication.

: What is "Encounter" in viral infection?

Infection can be acquired from other humans or the environment through direct contact (sexual contact, vertical transmission) or environmental routes (respiratory, gastrointestinal, transcutaneous)

What is "Entry" in viral infection?

The delivery of the viral genome into the cell, where it encodes proteins to control the cell and replicate.

: How can viral infection spread?

Through nerves to the central nervous system or through the blood to many organs. Some viruses use multiple pathways to spread.

What is the replication process of a viral infection?

Viral infection is often characterized by an incubation period, during which the virus replicates within the host before disease symptoms become evident.

What are the types of damage caused by viral infections?

Damage can be acute, latent, or chronic, and may be due to direct effects of viral replication or the host's response to the infection.

How is a viral infection diagnosed?

Through virus isolation in tissue culture, identification of viral antigens or nucleic acids, or detection of antiviral antibodies.

How can some viral diseases be treated?

Through specific antiviral medications.

How can some viral diseases be prevented or attenuated?

: By immunizations.

What is a virus particle, or virion, compared to?

A delivery system that surrounds a payload.

What is a naked virus?

A virion with a complete virus particle, consisting of a nucleic acid core (DNA or RNA), nucleoproteins, a protein coat (capsid), nucleocapsid, and spikes/viral attachment proteins (VAPs) for protection.

What are the components of a naked virus virion

Nucleic acid core (DNA or RNA), nucleoproteins, protein coat (capsid), nucleocapsid, and spikes/viral attachment proteins (VAPs).

What are the characteristics of the capsid structure?

It is a rigid structure that can withstand harsh environmental conditions, including resistance to drying, acid, and detergents, such as those found in the enteric tract.

How can viruses with a capsid structure be transmitted?

They can be transmitted by the fecal-oral route.

What is a viral attachment protein (VAP)?

A spike or fiber that helps the virus attach to host cells.

What is the capsid of a virus?

A single- or double-layer protein shell surrounding the viral nucleic acid. The nucleic acid and capsid are often referred to as the nucleocapsid.

What are capsids composed of?

Capsids are composed of subunits (protomer and capsomers) arranged in symmetric patterns.

How do capsid subunits interact?

: Each capsid subunit has the capacity to bind to other subunits in specific ways, allowing them to self-assemble to form the virus capsid (virion).

What happens if spikes or VAPs are removed or disrupted?

It inactivates the virus.

How can antibodies against VAPs help with viral infections?

Antibodies generated against the VAP can prevent virus infection.

What does an enveloped virus consist of?

A lipid envelope (lipid bi-layer) with protein projections, glycoproteins, and viral attachment proteins (VAPs).

Why is the envelope of an enveloped virus important?

The envelope is essential for infectivity.

What are the characteristics and transmission features of a naked capsid virus?

It is stable, released by cell lysis, easily spread (including via fomites), has a longer survival time, and may survive in the gut.

What are the characteristics and transmission features of an enveloped virus?

It is labile, released by budding or cell lysis, must remain wet, transmitted through close contact or droplets, and is destroyed in the gut.

: What are the different types of symmetry in a virus capsid?

Icosahedral (or spherical), helical, and complex.

What is the difference between symmetry and morphology in viruses?

Symmetry refers to the specific arrangement of the virus's capsid subunits (e.g., icosahedral, helical, complex), while morphology refers to the overall shape and structure of the virus particle.

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What is an example of a virus with helical symmetry, and what are its characteristics?

An example is the Tobacco Mosaic Virus (TMV), which is non-enveloped, has a capsid, and an RNA helix with associated nucleoproteins.

What are the characteristics of icosahedral symmetry in viruses?

It has repeating sub-units, requires few proteins, sub-units can self-assemble, is efficient, has the largest volume possible, and is very strong.

What are complex virus structures, and how are they defined?

Complex virus structures have a more intricate design, with parts of the virus shell built using general principles of symmetry. They cannot be simply defined by a mathematical equation.

What is unique about the structure of the T4 bacteriophage?

The T4 bacteriophage has both helical and icosahedral structures.

What type of symmetry does the poxvirus have?

The poxvirus has complex symmetry.

What are the different shapes of viruses, and what technology allowed their visualization?

Viruses can be icosahedral, spherical, rod-like, bullet-shaped, or filamentous. The development of the electron microscope allowed visualization of virus particles.

What are the characteristics of DNA in viruses?

DNA is a stable, larger molecule with a size range of 3 – 300 kbp. It can be dsDNA (circular or linear), pdsDNA (partially double-stranded, partially single-stranded circular DNA), or ssDNA (linear).

What are the characteristics of RNA in viruses?

RNA is easily degraded, smaller (3 - 30 kbp), and can be dsRNA (segmented), (+) ssRNA, (-) ssRNA, (-) ssRNA segmented, or (+/-) ambisense

What are the properties of a naked capsid virus and how is it released?

A naked capsid virus is composed of protein and is environmentally stable to temperature, acid, proteases, detergents, and drying. It is released from the cell by lysis. These properties affect the spread of the virus

What are the consequences of the properties of naked capsid viruses on disease spread?

Naked capsid viruses can spread easily (on fomites, by hand-to-hand contact, dust, or small droplets), dry out while retaining infectivity, survive adverse gut conditions, resist detergents and poor sewage treatment, and may be protected by antibodies.

What are the properties of enveloped viruses and how do they affect disease spread?

Enveloped viruses have components like membrane, lipids, proteins, and glycoproteins. They are environmentally labile, disrupted by acid, detergents, drying, and heat. The virus modifies the cell membrane during replication and is released by budding and cell lysis. These properties affect how the virus spreads.

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What are the consequences of the properties of enveloped viruses on disease spread?

Enveloped viruses must stay wet, cannot survive the gastrointestinal tract, and spread via large droplets, secretions, organ transplants, and blood transfusions. They do not need to kill the cell to spread and may require both antibody and cell-mediated immune responses for protection. These viruses can also cause hypersensitivity and inflammation, leading to immunopathogenesis.

What are some viruses associated with the respiratory system?

Influenza Virus, Respiratory Syncytial Virus (RSV), Parainfluenza Viruses, Common Cold, SARS-CoV-2, B19 Parvovirus, and Coronavirus.

What are some viruses associated with the reproductive system?

Herpes Simplex Virus (Both 1 & 2), Human Papillomavirus, Human Immunodeficiency Virus, and Molluscum Contagiosum Virus.

What are some viruses associated with viral skin diseases?

HSV 1 & 2, Varicella-Zoster Virus, Human Papillomavirus, B19 Parvovirus, Roseola, Molluscum Contagiosum Virus, and Enterovirus (Picorna).

What are some viruses associated with nervous system infections?

Enteroviruses, encephalitic arboviruses & roboviruses (e.g., WNV, SLE, WEE, EEE, RVF), Zika Virus, Rabies, Herpesviruses (HSV, VZV, MBV), JC virus, HIV, Reyes syndrome, and slow viral diseases (e.g., prions).