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Transmission dynamics of infectious diseases
The basic reproduction number (R0) is the number of infections caused by one case in a fully susceptible population, and it depends on the chance of transmission per contact, the number of contacts and the duration of infectiousness – an epidemic declines when the effective reproduction number falls below 1.
Why it matters in the exam
Items on this topic test definitions – endemic, epidemic and pandemic (defined by wide geographic spread across countries and continents, not by severity), incubation period versus latent period, carriage, serial interval and secondary attack rate – and applied questions: when the risk of spreading an infectious disease rises, how a patient's age relates to the duration of influenza shedding, and how each component of R0 translates into control measures. Masks and hand washing reduce the chance of transmission per contact, distancing and isolation reduce the number of contacts, and prompt treatment shortens infectiousness; vaccination reduces the susceptible share of the population. Further items deal with the chain of infection (agent, reservoir, portal of exit, mode of transmission, portal of entry and susceptible host) and the epidemiological triad of agent, host and environment. A key point is transmission before symptoms: when it is common, isolating patients alone is not enough, and contact tracing and quarantine are also needed. It is also worth distinguishing modes of transmission: direct contact, droplet, airborne, food and water, and vectors such as mosquitoes and sandflies; each calls for different preventive measures.
Key points
- R0 depends on the chance of transmission per contact, the number of contacts and the duration of infectiousness.
- An epidemic declines when the effective reproduction number falls below 1.
- A pandemic is defined by wide geographic spread.
- Transmission before symptoms calls for contact tracing, not only isolation.
- Breaking one link in the chain of infection stops transmission.
Summary table
| Concept | Definition | Meaning for control |
|---|---|---|
| R0 | Infections from one case in a fully susceptible population | Chance per contact times number of contacts times duration of infectiousness |
| Effective reproduction number (Re) | R0 times the susceptible share of the population | Below 1 – the epidemic declines |
| Incubation period versus latent period | Until symptoms, versus until infectiousness begins | If the latent period is shorter than incubation – transmission before symptoms |
| Serial interval | Time between symptoms in an infector and in the person infected | Sets the speed of spread |
| Endemic, epidemic, pandemic | Constant frequency; rise above expected; spread across countries and continents | A pandemic is defined by geographic extent, not severity |
| Chain of infection | Agent, reservoir, exit, transmission, entry, susceptible host | Breaking one link stops transmission |
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Sources
- Giesecke, Modern Infectious Disease Epidemiology, 3rd ed., 2017 — Basic concepts; Mathematical models
- CDC, Principles of Epidemiology in Public Health Practice, 3rd ed., 2012 — Introduction to epidemiology – chain of infection; Level of disease
- Heymann (ed.), Control of Communicable Diseases Manual, 21st ed., 2022 — Definitions
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.