Outbreaks

An outbreak is the occurrence of more cases of a disease than expected in a defined population over a specific period. Outbreaks can vary in size and duration, from a small cluster in a single institution to large clusters spanning regions. An outbreak is identified by a rise above baseline levels rather than by the presence of a pathogen alone.

Outbreaks are commonly categorized by transmission pattern. Point-source outbreaks arise from a single exposure over a short time, such as a contaminated food batch. Common-source outbreaks involve ongoing exposure to the same source. Propagated outbreaks result from person-to-person transmission and may spread over time. Outbreaks can also be zoonotic (originating in animals) or vector-borne (transmitted by insects), and may involve contaminated water or food supplies.

Public health surveillance detects potential outbreaks through reports from clinicians, laboratories, hospitals, and sentinel sites. An outbreak investigation typically follows steps to verify the diagnosis, define a case, identify and count cases, describe them by person, place, and time, generate hypotheses, test them with analytical studies, implement control measures, and monitor outcomes.

Control measures aim to reduce transmission and protect exposed populations. They may include isolation and treatment of cases, quarantine of contacts, vaccination or prophylaxis when available, recalls of contaminated products, environmental cleaning, advisories on water and food safety, and risk communication to the public. Effective response relies on rapid data collection, interagency coordination, and clear communication.

Historical examples include cholera outbreaks in the 19th century, the SARS outbreak in 2003, and subsequent Ebola outbreaks in West Africa (2014–2016) and Central Africa (2018–2020). Measles and foodborne outbreaks continue to occur in regions with gaps in vaccination and surveillance.