Pandemic Watch
Public-health glossary
13 hand-written definitions of the terms that appear most often in WHO Disease Outbreak News bulletins and CDC technical reports. Designed for journalists, students, and the curious public — clinically accurate but not clinician-only.
- Basic reproduction number R0
The expected number of secondary cases produced by a single infected individual in a fully susceptible population — a measure of how transmissible a pathogen is in its 'natural' state.
- Case definition
The standardized set of clinical, laboratory, and epidemiological criteria used to classify whether a given individual counts as a case of a specific disease for surveillance purposes.
- Case-fatality ratio CFR
The proportion of confirmed cases of a disease that result in death, expressed as a percentage. A measure of severity, not transmissibility.
- Disease Outbreak News DON
The WHO's official, publicly-available reporting channel for outbreak events of international concern, published with consistent numbering and structure since 1996.
- Endemic disease endemic
A disease that is constantly present at a predictable baseline level in a population or area — the normal expected level against which outbreaks and epidemics are measured.
- Epidemic threshold alert threshold
The number of cases at which disease activity crosses from the expected baseline into epidemic territory — the trigger point that mobilizes an outbreak response.
- Epidemic vs pandemic epidemic
An epidemic is a sharp rise in disease cases above the expected baseline within a community or region; a pandemic is an epidemic that has spread across multiple countries or continents. The difference is geographic scale, not severity.
- Outbreak disease outbreak
The occurrence of disease cases in excess of what is normally expected in a defined community, geographical area, or season — the operational threshold that triggers public-health response.
- Public Health Emergency of International Concern PHEIC
A formal designation under the International Health Regulations (IHR) declaring that an extraordinary disease event poses a public-health risk to other states and warrants a coordinated international response.
- Reservoir host natural reservoir
The animal species in which a pathogen normally lives and replicates, often without producing illness in that species — and from which human spillover events occur.
- Secondary attack rate SAR
The proportion of close contacts of a case who develop the disease within the incubation period — a real-world measure of person-to-person transmissibility in a defined setting.
- Vector-borne disease vector-borne illness
An infectious disease transmitted to humans by an arthropod intermediate — typically mosquitoes, ticks, fleas, or flies — rather than by direct contact with an animal reservoir.
- Zoonotic disease zoonosis
An infectious disease that can be transmitted between animals and humans. The majority of emerging human pathogens, including the four covered on this site, are zoonotic.
