A disease outbreak occurs when the number of people with a specific illness is higher than expected for a community, geographic area, or season. A disease outbreak can also refer to a group of cases with a common cause, such as infection with the same infectious agent or food poisoning. Outbreaks can be caused by infectious agents that spread directly from person to person, or they can be caused by contaminated items, such as needles or mosquitoes.

Infectious disease outbreaks can have significant social impacts. They can disrupt a region’s economy, disrupt travel and trade, and cause fear. People often flee their homes in response to disease outbreaks, resulting in sudden population movement.

Identifying and responding to outbreaks early can minimize their impact. In addition, public health interventions can help mitigate the risk of large-scale epidemics or pandemics, such as the one that occurred in 2014 with Ebola virus disease (EVD) in western Africa.

Successful outbreak investigations contribute to the development of knowledge about infectious diseases. For example, outbreak analysis has helped reveal new modes of transmission for well-known infectious pathogens. For example, while E coli O157:H7 infection was previously associated with eating undercooked hamburger meat, outbreak investigations have documented that the pathogen can also be transmitted through drinking unpasteurized milk, swimming in lakes and public water supplies, and through person-to-person contact. Outbreak investigations have also provided information about the spread of EVD to geographical areas where it was previously unknown, such as with immigrants and travellers.