Questions

Viral Transmission — Questions

Study questions for Viral Transmission.

Mock Exam mode

Sit this set one question at a time. Multiple-choice questions mark themselves; written questions reveal a tickable mark scheme so you can score your own answer. You get a combined score at the end.

18 questions: 18 MCQ, 0 written.

A virus that relies on vertical transmission needs the host to survive and reproduce, so selection pushes it toward lower virulence, since killing or sterilising the host is a reproductive dead end.

Higher virulence is more readily tolerated by horizontally transmitted viruses, which do not depend on the host’s reproductive success to reach a new host.

Virus produced at a site with no route to a body surface reaches a transmission dead end and is not shed, even when the infection there is severe or lethal.

Shedding requires a portal that communicates with the outside, which the brain and most internal organs do not provide.

Enveloped respiratory viruses are labile and survive poorly outside the body, so the respiratory route depends on relatively close or repeated contact.

They do not persist on surfaces or in water the way hardy, non-enveloped enteric viruses do.

For most viruses the site of shedding mirrors the site of entry, so a virus entering by the respiratory tract is generally shed from it, and a gut virus is shed in faeces.

Generalised infections are the exception, shedding from several sites at once, as cytomegalovirus does into saliva, urine, semen and milk.

Both are healthcare-associated, but iatrogenic transmission is spread specifically by a medical procedure or practitioner, the 1976 Ebola outbreak in Zaire, amplified by reused unsterilised needles, being the extreme example.

Nosocomial transmission is the broader category of acquiring a virus in a hospital or clinic.

Comprehensive donor screening has made transfusion transmission rare, so the blood-borne viruses now spread mainly through shared injecting equipment.

They are not transmitted by the respiratory, faecal-oral or casual-contact routes, and unbroken skin is an effective barrier.

Overdispersion means transmission is uneven, with a small number of cases or events causing most secondary infections, the superspreading pattern seen with SARS and SARS-CoV-2.

It makes an epidemic’s early course erratic and means control focused on high-transmission settings can be disproportionately effective.

SARS in 2003 had little pre-symptomatic transmission and a generation time of about a week, so isolating symptomatic cases and quarantining their contacts interrupted spread.

A virus that transmitted mostly before symptoms appeared would defeat the same symptom-based measures.

R₀ is the product of three components: how efficiently the virus transmits at each contact, how many contacts an infected person makes, and how long that person stays infectious.

The other options combine quantities that do not compose the reproduction number.

After a vector takes an infected blood meal, the extrinsic incubation period is the time the virus multiplies and reaches the salivary glands before the vector can transmit it, and it shortens at higher ambient temperature.

The intrinsic incubation period, by contrast, is the interval that runs in the human host after the bite.

Infectivity is quantified by the dose that infects half of those exposed, the ID50.

Virulence, by contrast, is the probability of severe disease and is read from the case-fatality rate; the reproduction number and shedding duration measure different things again.

The classical droplet-versus-aerosol cut-off is best read as an approximation, because respiratory spread sits on a continuum rather than splitting cleanly into two mechanisms, a view reinforced by the reappraisal that followed SARS-CoV-2.

Fine aerosols can travel beyond a metre or two and linger in the air, so the boundary is not sharp.

Transovarial transmission passes the virus to the vector’s next generation through the egg, which helps some arboviruses survive between transmission seasons.

It is distinct from replication in the vector, from mechanical carriage on the mouthparts, and from biological transmission to a human host.

Faecal-oral viruses are typically non-enveloped and survive in water and on surfaces for days to weeks, which is what lets them spread through contaminated food, water and hands.

Enveloped viruses, by contrast, are labile in the environment and do not persist well enough to use this route.

When the latent period is shorter than the incubation period, a person becomes infectious before symptoms appear, so transmission is pre-symptomatic, as it is for measles and chickenpox.

This blunts any control measure that relies on detecting symptoms before a person can spread the virus.

Vertical transmission occurs by one of three mechanisms: integration into the germline, transplacental spread in pregnancy, and perinatal or postnatal spread at delivery and through breast milk.

The other routes listed are all horizontal, passing the virus between individuals in a population.

Vertical transmission passes a virus from parent to offspring, in utero, around birth, or through breast milk, so mother-to-infant HIV at delivery is the vertical example.

The others are all horizontal transmission, between individuals in a population, by the respiratory, faecal-oral, contact and vector routes.

Indirect transmission bridges a gap through an intermediary, so a contaminated fomite such as a door handle is the indirect route.

The others are forms of direct transmission, which need immediate contact between an infected and a susceptible host.

MCQ
A virus that depends heavily on vertical transmission is under selective pressure to:
- A. Evolve toward lower virulence
- B. Maximise host mortality
- C. Sterilise its host
- D. Abandon its host entirely
- E. Depend on an arthropod vector
Show answer
Correct answer: A
A virus that relies on vertical transmission needs the host to survive and reproduce, so selection pushes it toward lower virulence, since killing or sterilising the host is a reproductive dead end.

Higher virulence is more readily tolerated by horizontally transmitted viruses, which do not depend on the host’s reproductive success to reach a new host.
MCQ
A virus that replicates to high titre in the brain, at a site with no route to a body surface, will:
- A. Be shed efficiently in the saliva and respiratory secretions
- B. Be excreted in the urine
- C. Spread readily to close contacts
- D. Pass easily to the next host
- E. Reach a dead end and not be transmitted
Show answer
Correct answer: E
Virus produced at a site with no route to a body surface reaches a transmission dead end and is not shed, even when the infection there is severe or lethal.

Shedding requires a portal that communicates with the outside, which the brain and most internal organs do not provide.
MCQ
Enveloped respiratory viruses depend on relatively close or repeated contact mainly because they:
- A. Are labile and survive poorly outside the host
- B. Require an arthropod vector
- C. Are shed only in the faeces
- D. Infect exclusively the lower airway
- E. Need a contaminated water source to spread between hosts
Show answer
Correct answer: A
Enveloped respiratory viruses are labile and survive poorly outside the body, so the respiratory route depends on relatively close or repeated contact.

They do not persist on surfaces or in water the way hardy, non-enveloped enteric viruses do.
MCQ
For most viruses, the site from which infectious virus is shed:
- A. Mirrors the site of entry
- B. Is usually the bloodstream
- C. Bears no relation to the site of entry
- D. Is confined to the skin surface
- E. Is limited to the urinary tract
Show answer
Correct answer: A
For most viruses the site of shedding mirrors the site of entry, so a virus entering by the respiratory tract is generally shed from it, and a gut virus is shed in faeces.

Generalised infections are the exception, shedding from several sites at once, as cytomegalovirus does into saliva, urine, semen and milk.
MCQ
Iatrogenic transmission is best distinguished from nosocomial transmission in that:
- A. It happens only outside recognised healthcare settings
- B. It is spread by a medical procedure or practitioner
- C. It is limited to respiratory viruses
- D. It spares immunocompromised patients
- E. It requires an animal reservoir
Show answer
Correct answer: B
Both are healthcare-associated, but iatrogenic transmission is spread specifically by a medical procedure or practitioner, the 1976 Ebola outbreak in Zaire, amplified by reused unsterilised needles, being the extreme example.

Nosocomial transmission is the broader category of acquiring a virus in a hospital or clinic.
MCQ
In most countries today, blood-borne viruses such as hepatitis C are transmitted mainly by:
- A. Blood transfusion from unscreened donors
- B. Shared injecting equipment
- C. Respiratory droplets
- D. The faecal-oral route
- E. Casual household contact
Show answer
Correct answer: B
Comprehensive donor screening has made transfusion transmission rare, so the blood-borne viruses now spread mainly through shared injecting equipment.

They are not transmitted by the respiratory, faecal-oral or casual-contact routes, and unbroken skin is an effective barrier.
MCQ
Overdispersion in transmission, the superspreading pattern, means that:
- A. Every case infects a similar number of others
- B. Transmission cannot be modelled
- C. A few cases cause most secondary infections
- D. The virus lacks any reservoir
- E. Herd immunity is impossible to reach
Show answer
Correct answer: C
Overdispersion means transmission is uneven, with a small number of cases or events causing most secondary infections, the superspreading pattern seen with SARS and SARS-CoV-2.

It makes an epidemic’s early course erratic and means control focused on high-transmission settings can be disproportionately effective.
MCQ
Quarantine and isolation contained the 2003 SARS epidemic largely because the virus had:
- A. A very high fraction of transmission occurring before any symptoms appear
- B. No capacity for human-to-human spread
- C. An effective vaccine deployed within weeks
- D. Little pre-symptomatic transmission and a generation time of about a week
- E. An animal reservoir that was culled
Show answer
Correct answer: D
SARS in 2003 had little pre-symptomatic transmission and a generation time of about a week, so isolating symptomatic cases and quarantining their contacts interrupted spread.

A virus that transmitted mostly before symptoms appeared would defeat the same symptom-based measures.
MCQ
The basic reproduction number, R₀, can be understood as the product of:
- A. Transmissibility per contact, the contact rate, and the infectious duration
- B. The incubation period and the overall case-fatality rate
- C. The prevalence and the incidence of infection
- D. Vaccine coverage and the herd-immunity threshold
- E. The latent period and the observed serial interval measured between successive cases
Show answer
Correct answer: A
R₀ is the product of three components: how efficiently the virus transmits at each contact, how many contacts an infected person makes, and how long that person stays infectious.

The other options combine quantities that do not compose the reproduction number.
MCQ
The extrinsic incubation period of an arbovirus is:
- A. The interval from human infection to the onset of symptoms
- B. The time the virus multiplies in the vector before transmission
- C. The total lifespan of the adult arthropod vector
- D. The period a human host remains infectious to others
- E. The delay before specific antibodies become detectable in the blood
Show answer
Correct answer: B
After a vector takes an infected blood meal, the extrinsic incubation period is the time the virus multiplies and reaches the salivary glands before the vector can transmit it, and it shortens at higher ambient temperature.

The intrinsic incubation period, by contrast, is the interval that runs in the human host after the bite.
MCQ
The infectivity of a virus, the probability of infecting a susceptible host on exposure, is quantified by:
- A. The case-fatality rate
- B. The basic reproduction number
- C. The incubation period
- D. The total duration of viral shedding after infection
- E. The dose that infects half of those exposed
Show answer
Correct answer: E
Infectivity is quantified by the dose that infects half of those exposed, the ID50.

Virulence, by contrast, is the probability of severe disease and is read from the case-fatality rate; the reproduction number and shedding duration measure different things again.
MCQ
The traditional size cut-off separating respiratory droplets from aerosols is best regarded as:
- A. A precise, fixed physical boundary
- B. Proof that fine aerosols play no role in transmission
- C. Irrelevant to how viruses spread
- D. An approximation, since spread lies on a continuum
- E. Evidence against airborne transmission
Show answer
Correct answer: D
The classical droplet-versus-aerosol cut-off is best read as an approximation, because respiratory spread sits on a continuum rather than splitting cleanly into two mechanisms, a view reinforced by the reappraisal that followed SARS-CoV-2.

Fine aerosols can travel beyond a metre or two and linger in the air, so the boundary is not sharp.
MCQ
Transovarial transmission in an arthropod vector refers to:
- A. Mechanical carriage of the virus on contaminated mouthparts
- B. Virus replication within the vector's salivary glands
- C. Loss of the virus from the vector at each larval moult
- D. Passage of the virus to the next generation through the egg
- E. Spread between two adjacent human hosts by a single feeding insect
Show answer
Correct answer: D
Transovarial transmission passes the virus to the vector’s next generation through the egg, which helps some arboviruses survive between transmission seasons.

It is distinct from replication in the vector, from mechanical carriage on the mouthparts, and from biological transmission to a human host.
MCQ
Viruses transmitted by the faecal-oral route are typically:
- A. Enveloped and rapidly inactivated in the environment
- B. Dependent on an arthropod vector
- C. Non-enveloped and environmentally stable
- D. Unable to survive outside the host
- E. Spread mainly by aerosol
Show answer
Correct answer: C
Faecal-oral viruses are typically non-enveloped and survive in water and on surfaces for days to weeks, which is what lets them spread through contaminated food, water and hands.

Enveloped viruses, by contrast, are labile in the environment and do not persist well enough to use this route.
MCQ
When a virus's latent period is shorter than its incubation period:
- A. The virus cannot be transmitted
- B. Symptoms reliably appear before infectiousness begins in the host
- C. Quarantine becomes fully effective
- D. The infection is usually subclinical
- E. A person becomes infectious before feeling ill
Show answer
Correct answer: E
When the latent period is shorter than the incubation period, a person becomes infectious before symptoms appear, so transmission is pre-symptomatic, as it is for measles and chickenpox.

This blunts any control measure that relies on detecting symptoms before a person can spread the virus.
MCQ
Which of the following is a recognised mechanism of vertical transmission?
- A. Aerosol inhalation from an infected contact
- B. Faecal-oral ingestion
- C. Transplacental spread in pregnancy
- D. A mosquito bite
- E. Sexual contact between adults
Show answer
Correct answer: C
Vertical transmission occurs by one of three mechanisms: integration into the germline, transplacental spread in pregnancy, and perinatal or postnatal spread at delivery and through breast milk.

The other routes listed are all horizontal, passing the virus between individuals in a population.
MCQ
Which of the following is an example of vertical transmission?
- A. Influenza passed by coughing between colleagues
- B. Hepatitis A acquired from contaminated water
- C. HIV passed from a mother to her infant at birth
- D. Norovirus picked up from a contaminated surface
- E. Rabies acquired from the bite of an infected animal
Show answer
Correct answer: C
Vertical transmission passes a virus from parent to offspring, in utero, around birth, or through breast milk, so mother-to-infant HIV at delivery is the vertical example.

The others are all horizontal transmission, between individuals in a population, by the respiratory, faecal-oral, contact and vector routes.
MCQ
Which of the following represents indirect transmission?
- A. Skin-to-skin contact between two people
- B. A short-range respiratory droplet
- C. Sexual contact between partners
- D. A door handle contaminated with virus
- E. The bite of an infected animal
Show answer
Correct answer: D
Indirect transmission bridges a gap through an intermediary, so a contaminated fomite such as a door handle is the indirect route.

The others are forms of direct transmission, which need immediate contact between an infected and a susceptible host.