Chapter 8

What are viruses?

Submicroscopic infectious agents that are nonliving pathogens.

What is virology?

The study of viruses.

Approximately what percentage of human-infecting viruses are harbored in other animals?

About 70%.

What are the three main characteristics of viruses?

Extremely small, acellular, and obligate intracellular pathogens.

What does "obligate intracellular pathogen" mean?

A pathogen that must live and reproduce inside a host cell.

What is a bacteriophage?

A virus that infects bacteria.

What is an animal virus?

A virus that infects animals and humans.

What is a virion?

A single infectious virus particle.

What are the two main components of a virion?

Genetic material and a capsid.

What is a capsid?

The protein shell that surrounds and protects the viral genome.

What protein subunits make up a capsid?

Capsomeres.

What are the two most common capsid shapes in animal viruses?

Helical and icosahedral.

What does a helical capsid look like?

A hollow tube.

What does an icosahedral capsid look like?

A three-dimensional polygon.

What type of capsid do bacteriophages have?

Complex capsids.

Why do bacteriophages have specialized structures?

To inject their genome into host cells.

What is a viral envelope?

A lipid membrane surrounding the capsid.

How do enveloped viruses obtain their envelopes?

By budding from the host cell membrane.

What are naked viruses?

Viruses that lack an envelope.

How are naked viruses typically released?

By lysing (bursting) the host cell.

Can animal viruses be naked or enveloped?

Yes.

Why are bacteriophages always naked?

Because they are released by cell lysis.

What are viral spikes (peplomers)?

Glycoprotein projections used for attachment and entry into host cells.

What are the two major spike proteins of Influenza A?

Hemagglutinin (HA) and Neuraminidase (NA).

What do viral genes encode?

Capsid proteins, replication enzymes, and structural factors.

Can viral genomes be DNA or RNA?

Yes.

Can viral genomes be single- or double-stranded?

Yes.

Can viral genomes be circular or linear?

Yes.

What is the ultimate goal of every virus?

To get the host cell to produce viral proteins and new virions.

How do dsDNA viruses make proteins?

DNA is transcribed into mRNA, which is translated into protein.

What must happen before ssDNA viruses can be transcribed?

They must be converted into double-stranded DNA.

How does positive-sense ssRNA function?

It acts directly as mRNA.

What enzyme do retroviruses use to make DNA from RNA?

Reverse transcriptase.

What happens to retroviral DNA after it is made?

It is inserted into the host genome.

What enzyme is required by dsRNA viruses?

RNA-dependent RNA polymerase.

Why do viruses evolve rapidly?

They replicate quickly and RNA viruses mutate frequently.

Do RNA polymerases have proofreading abilities?

No.

What are attenuated viral strains?

Weakened strains used in vaccines.

What advantages can beneficial mutations provide viruses?

Immune escape, broader host range, expanded tropism, and increased infectivity.

What is reassortment?

Mixing of genetic material when two viral strains infect the same cell.

What is antigenic drift?

Small mutations in influenza HA and NA spike proteins.

What is antigenic shift?

Major genetic reassortment producing a new influenza strain.

Why is antigenic shift dangerous?

It can cause pandemics because people lack immunity.

What organization develops virus naming criteria?

The International Committee on Taxonomy of Viruses.

What characteristics are used to classify viruses?

Nucleic acid type, capsid symmetry, envelope presence, and genome architecture.

What is the highest taxonomic rank used for viruses?

Order.

What taxonomic levels come below order?

Family, genus, species.

What is host range?

The collection of species a virus can infect.

What is tropism?

The specific cells or tissues a virus infects.

 What is broad tropism?

Ability to infect many cell types.

What is narrow tropism?

Ability to infect only one cell type or tissue.

How small can viruses be?

About 30 nm.

Which viruses are among the smallest?

Rhinoviruses and polioviruses.

Which virus is among the largest known viruses?

Pithovirus.

What is a prophage?

Phage DNA integrated into the bacterial genome.

What happens during lysogenic replication?

The prophage is copied as the host cell divides.

What can trigger entry into the lytic cycle?

Host cell stress.

 What are the five steps of bacteriophage replication?

Attachment, penetration, replication, assembly, and release.

What occurs during attachment?

The phage binds to the bacterial cell.

What occurs during penetration?

The phage injects its genome.

What occurs during replication?

Viral DNA and proteins are synthesized.

What occurs during assembly?

Viral genomes are packaged into capsids.

 What occurs during release?

The host cell lyses and releases new phages.

What is an acute infection?

A short-term infection producing new virions.

What is a persistent infection?

An infection that avoids immune clearance.

What is a chronic persistent infection?

Continuous viral release over months or years.

What is a provirus?

Viral DNA integrated into a host genome.

What is an example of a chronic viral infection?

HIV.

What is latency?

A dormant period with no active viral production.

What happens during a flare-up?

Symptoms appear and virions are shed.

What can trigger latent virus flare-ups?

Stress, fever, sunburn, or hormone changes.

Which virus family is known for latent infections?

Herpesviridae.

What disease is caused by HSV-1?

Cold sores.

What disease is caused by HSV-2?

Genital herpes.

What diseases are caused by HHV-3?

Chickenpox and shingles.

What are oncogenic viruses?

Viruses capable of causing cancer.

Why are viruses grown in laboratories?

To develop vaccines and antiviral drugs.

What is required to grow a virus?

A host cell.

What is a plaque?

A clear zone where infected cells have lysed.

 What is a plaque-forming unit (PFU)?

A measure of infectious bacteriophages.

What is viral titer?

The amount of virus in a sample.

How are most animal viruses grown?

Using tissue culture techniques.

What other systems can be used to grow animal viruses?

Live animals and embryonated eggs.

Why are molecular methods commonly used to detect viruses?

Viruses cannot usually be seen with a standard light microscope.

What does specificity mean?

No false positives.

What does sensitivity mean?

No false negatives.

How are viral proteins commonly detected?

Using antibodies.

What happens in an agglutination test?

Antibody-coated beads clump together when viral antigens are present.

What is a latex agglutination test?

A test using latex beads coated with antibodies or antigens.

What does ELISA stand for?

Enzyme-Linked Immunosorbent Assay.

What can ELISA detect?

Antigens or antibodies.

What indicates a positive ELISA result?

A color change.

Why are nucleic acid tests popular?

They are highly sensitive and often rapid.

What are three common methods for detecting viral nucleic acids?

PCR, sequencing, and fluorescent probes.

What samples may be used for nucleic acid testing?

Blood, sputum, or CSF.

Do antiviral drugs usually cure viral infections?

No, they usually limit infection.

Why is it difficult to design antiviral drugs?

Viruses use host-cell machinery and have few unique targets.

Why is vaccination important?

It prevents serious viral diseases and trains the immune system.

What is postexposure prophylaxis?

Injectable antibodies given shortly after exposure.

What diseases commonly use postexposure prophylaxis?

Rabies and HIV.