Viral Epidemiology

Epidemiology is the study of the distribution, dynamics and determinants of disease in populations. Applied to viruses it does two jobs at once: it measures where and how fast infection occurs, and it explains why, giving the quantitative basis for control. Because a virus cannot replicate outside a living cell, its survival in nature depends on maintaining an unbroken chain of transmission from one host to the next, and much of viral epidemiology follows from that single constraint.

The topic is treated in two articles. The first covers the measurement and investigation side: how disease frequency is quantified, how epidemiological studies establish cause, and the population dynamics that decide whether a virus endures. The second covers transmission itself: how viruses are shed, the routes they travel, and the determinants of how efficiently they spread.

→ See Viral Epidemiology for the measures of disease frequency, patterns of occurrence, epidemiological study designs, sero- and molecular epidemiology, the basic reproduction number and herd immunity, and how viruses persist in populations.

→ See Viral Transmission for the mechanisms of virus survival, viral shedding and the routes of transmission, the dynamics of spread, and the determinants of transmissibility.

Key terms

The core vocabulary of the topic, grouped by theme.

Classifying transmission:

Term	Definition
Horizontal transmission	Spread between individuals in a population, regardless of lineage; the commonest mode.
Vertical transmission	Passage from parent to offspring: germline, transplacental, perinatal, or through breast milk.
Direct transmission	Spread needing immediate contact between an infected and a susceptible host, such as skin or sexual contact, a bite, or a short-range respiratory droplet.
Indirect transmission	Spread bridged by an intermediary rather than direct contact.
Fomite	An inanimate object that carries the virus between hosts (indirect).
Vehicle	Contaminated food, water or a blood product that carries the virus (indirect).
Vector	A living organism, usually an arthropod, that carries the virus between hosts (indirect).

Measures of disease frequency:

Term	Definition
Incidence (attack rate)	New cases arising in a population over a defined period, per standard population and time.
Secondary attack rate	New cases among susceptible contacts of an index case within one incubation period, as a percentage of those exposed.
Prevalence	Cases present in a population at one point in time, divided by the population.
Seroprevalence	Proportion of a population with antibody to a virus; reflects cumulative past infection.
Case-fatality rate	Percentage of people with the disease who die of it.

Spread and persistence:

Term	Definition
Basic reproduction number (R₀)	Mean secondary infections from one case in a wholly susceptible population.
Effective reproduction number (R)	R₀ multiplied by the susceptible fraction; control holds it below 1.
Herd-immunity threshold	The immune proportion that stops spread, equal to 1 minus 1/R₀.
Generation time	Average interval between an infection and the infections it causes.
Critical community size	Minimum population needed to maintain an infection indefinitely.
Endemic, epidemic, pandemic	A steady baseline rate; a rise above it; spread across continents.

References and recommended reading

• Burrell CJ, Howard CR, Murphy FA. Epidemiology of Viral Infections. In: Fenner and White’s Medical Virology, 5th edition, Chapter 13. Academic Press / Elsevier; 2017. The foundational backbone for the measures, study designs, transmission and population dynamics of viral epidemiology.
• van Seventer JM, Hochberg NS. Principles of Infectious Diseases: Transmission, Diagnosis, Prevention, and Control. In: International Encyclopedia of Public Health, 2nd edition, Volume 6. Elsevier; 2017. p. 22–39. A public-health framework source for the transmission taxonomy, the reproduction number and herd immunity.