viral infection

Question 1

Properties of a Virus

Answer 1

Obligate intracellular parasites
Infect bacteria, protozoa, fungi, algae, plants, and animals
Protein capsid surrounds nucleic acid
Crystalline appearance
Sizes: 20 nm to 450 nm
Naked = nucleocapsid; Enveloped = nucleocapsid + envelope
Viral morphologies: Complex, Enveloped, Non-enveloped (Naked)

Question 2

Viral Classification

Answer 2

Classified by structure, chemical composition, and genetic makeup
All DNA viruses are double-stranded (except parvoviruses with ssDNA)
All RNA viruses are single-stranded (except dsRNA reoviruses)
DNA viruses bud off the nucleus
RNA viruses multiply in and are released from the cytoplasm

Question 3

General Features of Viral Replication

Answer 3

Absorption
Penetration
Uncoating
Synthesis (DNA/RNA dependent)
Assembly
Release

Question 4

Viral Entry: Modes of Penetration

Answer 4

Endocytosis: Virus enters via vesicle, vesicle breaks down, releases nucleic acid
Fusion: Viral membrane fuses with host cell, nucleocapsid enters, uncoating releases RNA

Question 5

Viral Pathogenesis

Answer 5

Viral infection → disease, often subclinical
Virus usually doesn’t benefit from harming/killing host
Outcomes: Acute infection, chronic infection, death, or reactivation
Chronic infections can lead to cancers (e.g., EBV, HPV, HBV)

Question 6

Viral Transmission

Answer 6

Respiratory: Influenza A
Faecal-oral: Enterovirus
Blood-borne: Hepatitis B
Sexual: HIV
Vectors: Rabies

Question 7

Cell Tropism

Answer 7

Viral affinity for specific tissues
Depends on cell receptors, transcription factors, physical barriers, local environment
Tropism determines replication and spread

Question 8

Immune Response to Viral Infection

Answer 8

Type I Interferons (IFN-α, IFN-β) triggered by viral dsRNA
NK cells activated, killing infected cells
Humoral response: antibodies block viral entry, neutralize, or opsonize viruses

Question 9

Viral Evasion of Host Defenses

Answer 9

Viruses block antigen presentation (MHC I, MHC II)
Evade complement-mediated destruction
Cause generalized immunosuppression
Antigenic variation (e.g., Influenza)

Question 10

Cell-Mediated Antiviral Mechanisms

Answer 10

CD8+ T-cells and CD4+ Th1 cells key in eliminating viral infection
Antibodies contain spread but cannot eliminate the virus once it infects cells

Question 11

What are the main differences between DNA and RNA viruses in terms of replication and release from the host cell?

Answer 11

DNA viruses often replicate in the nucleus and are released via budding. RNA viruses typically replicate in the cytoplasm and are released from there.

Question 12

Describe the structure of a virus and the key components that all viruses have.

Answer 12

All viruses have a nucleic acid (DNA or RNA) surrounded by a protein capsid. Some viruses also have an outer lipid envelope.

Question 13

What are the two main methods viruses use to enter a host cell?

Answer 13

Endocytosis and membrane fusion.

Question 14

What is viral tropism, and what factors determine it?

Answer 14

Viral tropism is the virus’s affinity for specific tissues, determined by cell receptors, transcription factors, physical barriers, and local environmental conditions.

Question 15

How do viruses typically evade the host immune system

Answer 15

They interfere with antigen presentation, reduce MHC molecule levels, evade complement-mediated destruction, or cause immunosuppression.

Question 16

What are some common outcomes of acute viral infections?

Answer 16

Recovery with or without residual effects, death, or progression to chronic infection.

Question 17

What distinguishes a chronic viral infection from an acute one?

Answer 17

Chronic infections may persist silently, reactivate periodically, or cause long-term diseases like cancers, while acute infections resolve more quickly.

Question 18

Give examples of viruses transmitted by the respiratory route, faecal-oral route, and vectors.

Answer 18

Respiratory: Influenza A. Faecal-oral: Enterovirus. Vectors: Rabies.

Question 19

What role do CD8+ T-cells and CD4+ Th1 cells play in combating viral infections

Answer 19

CD8+ T-cells directly kill infected cells, while CD4+ Th1 cells support immune responses by activating macrophages and producing cytokines.

Question 20

How do antibodies neutralize viruses, and at what stage of infection are they most effective?

Answer 20

Antibodies neutralize viruses by preventing attachment to host cells or blocking viral penetration. They are most effective during acute infections and reinfection.

Question 21

What are Type I interferons (IFN-α, IFN-β), and how do they help combat viral infections?

Answer 21

These interferons are produced in response to viral dsRNA and activate Natural Killer cells and protein kinases that inhibit viral replication.

Question 22

Explain the concept of viral latency and give an example of a virus that can reactivate after a long period.

Answer 22

Viral latency refers to a dormant state in which the virus persists without causing symptoms but can reactivate later, such as herpes or varicella-zoster (shingles).

Question 23

How does the immune system’s humoral response differ from the cell-mediated response during a viral infection?

Answer 23

The humoral response, involving antibodies, prevents viral spread and reinfection, while the cell-mediated response eliminates infected cells.

Question 24

What are the key factors that determine the course of a viral infection within a host?

Answer 24

Viral entry, tissue tropism, host immune response, viral replication strategy, and cellular damage.