Outbreak

An outbreak, in epidemiology, is the occurrence of cases of a disease above the expected baseline level for a given population, location, and time. The threshold is relative: a single case of a normally absent disease (Ebola in Belgium, polio in a country certified polio-free) constitutes an outbreak, while ten cases of common-cold rhinovirus does not. Outbreak detection depends on having a baseline to compare against, which is one reason why ongoing surveillance is the foundation of outbreak response.

The terms outbreak, epidemic, and pandemic exist on a spectrum of scale. WHO and CDC use 'outbreak' for localized events, 'epidemic' for sustained transmission in a larger area, and 'pandemic' for transmission across multiple countries or continents. The boundaries are not strictly defined — they are operational labels that reflect both the geographic scope and the response level being mobilized.

Outbreak response classically proceeds through the same seven steps: verify the diagnosis, define and count cases, describe the affected population in time/place/person, generate hypotheses about the source, test those hypotheses, implement control measures, and communicate findings. This framework was codified by the CDC's Epidemic Intelligence Service in the 1950s and remains the basis for outbreak response training globally.