CSIM1.16: Viruses and disease

Question 1

What are the 3 main modes of viral transmission?

Answer 1

1. Horizontal transmission (one human to another)
2. Vertical transmission (mum to offspring)
3. Zoonotic (animal to human)

Question 2

What are the 6 types of horizontal transmission?

Answer 2

Common:

1. Respiratory
2. Fecal

Less common:

3. Sexual
4. Mechanical (skin related cuts/abrasions)
5. Urine
6. Conjunctiva

Question 3

What are examples of respiratory transmission related viruses that can cause localised infections?

Answer 3

1. Rhinoviruses (common cold)

2. Influenza (flu)

Question 4

What are examples of respiratory transmission related viruses that can cause generalised (elsewhere) infections?

Answer 4

1. Chicken pox (varicella zoster virus)
2. Measles (paramyxoviridae)
3. Small pox (variola virus)

Question 5

What are the factors that influence the efficiency of respiratory transmission of viruses?

Answer 5

The size of the water droplets that humans emit: most efficient for transmission would be MIDDLE RANGE DROPLETS

LARGE DROPLETS (>10µm [micrometer ^(-6)]) rapidly fall to the ground

SMALL DROPLETS (

Question 6

What are some examples of viruses that get transmitted through the fecal-oral route?

Answer 6

Picornaviruses like

1. Polio
2. Hep A

Norovirus

Question 7

What makes faeces good virus carriers?

Answer 7

They contain 1000 infection units (IU) per microliter

Question 8

What does NOROVIRUS cause?

Answer 8

It is the most common cause of viral gastroenteritis

Question 9

What are some examples of viruses that get transmitted through vertical transmission?

Answer 9

1. Rubella (usually spread via respiratory route and if patient is pregnant, this crosses placenta causing foetal death/malformations)
2. HIV-1

Question 10

What are some examples of viruses that get transmitted through zoonotic transmission?

Answer 10

Usually by arthropods

Arboviruses like:

1. Dengue fever
2. Yellow fever
3. Japanese encephalitis

Question 11

How can a human infect another human indirectly with a virus that is transmitted via zoonosis?

Answer 11

An infected arthropod infects a human -> an uninfected arthropod bites infected human -> now infected arthropod bites healthy human

Question 12

What is pathogenicity?

Answer 12

The severity of the disease caused by DIFFERENT viruses

e.g. effects of RABIES vs INFLUENZA

Question 13

What is virulence?

Answer 13

The severity of disease caused by strains of the SAME virus.

e.g. 10 virions of HSV-1 A to kill a mouse vs 10000 virions of HSV-1 B to kill a mouse

Question 14

What are pathogenicity factors?

Answer 14

Characteristics viruses possess to allow them to be pathogenic

Question 15

What are pathogenic factors?

Answer 15

1. Entry
2. Initial replication
3. Immune evasion
4. Dissemination
5. Further replication
6. Shedding (infect others)

Question 16

Describe the pathogenicity of localised infections

Answer 16

Only go through entry -> initial replication -> shedding as infection is localised so sheds to localised area

Question 17

Describe the pathogenicity of generalised infections

Answer 17

Upon entry, primary viraemia occurs and on further multiplication, secondary viraemia occurs with further entry to bloodstream to finally cause onset of disease

e.g. Primary viraemia from blood to lymph nodes; secondary from blood to blood near brain causing rare encephalitis in measles

Question 18

What is incubation period?

Answer 18

The time between VIRAL EXPOSURE and ONSET OF ILLNESS (symptoms becoming apparent)

e.g. Localised infections have short incubation periods

Question 19

What is generation time?

Answer 19

Time between VIRAL EXPOSURE (infection) to ONSET OF INFECTIOUSNESS

Question 20

What is reproduction number (r0)?

Answer 20

The number of subsequent infections in a susceptible population caused by a viral illness in one individual during its INFECTIOUS PERIOD?

Question 21

What is a subclinical infection?

Answer 21

Asymptomatic

Question 22

What is a latent infection?

Answer 22

Virus cells lie dormant in host cells until immune system is compromised/suppressed [usually a slowly progressive disease like shingles after VPV]