Outbreak

An outbreak is the occurrence of cases of a disease or health condition in excess of what is normally expected in a defined community or setting over a defined period.

The term is often used for infectious diseases, but it can apply to noninfectious events such as clusters of foodborne illness, exposure to environmental hazards, or toxic substances. Outbreaks may be localized to a school, workplace, or neighborhood, and if spread broadly they may be described as epidemics or pandemics.

Detection and investigation: Public health surveillance detects unusual increases in illness. Confirmation involves establishing a case definition, counting cases, laboratory testing, and an epidemiologic investigation to identify the source, mode of transmission, and risk factors.

Response: Control measures include isolation and treatment of cases, contact tracing, quarantine where appropriate, vaccination or postexposure prophylaxis, environmental cleaning, water and food safety measures, and clear public communication. Effective outbreak response requires coordination across health departments, laboratories, clinicians, and sometimes international agencies.

Impact and examples: Notable outbreaks include the 2014-2016 West Africa Ebola outbreak, Legionnaires’ disease clusters, and seasonal influenza outbreaks. Outbreaks drive improvements in surveillance, diagnostics, rapid response, and public health policy, including reporting requirements and international health regulations.

In public health, outbreak investigation is a disciplined process aimed at stopping transmission, reducing morbidity and mortality, and preventing future occurrences through learning and system strengthening.