A public health emergency caused by a disease or other biological risk that spreads across a country or region and impacts on the general population. Epidemics are often triggered by global environmental changes, such as droughts, floods and earthquakes, which create conditions that favour the transmission of diseases. However, a range of social factors also contribute to their spread, such as marginalisation, disruption and conflict, creating conditions that exacerbate vulnerability and facilitate infection. The HIV/AIDS pandemic has shown how biological hazards can exploit social fault lines and create opportunities for distancing and stigma.
Epidemics occur when the number of people infected with a disease increases rapidly. They can be categorised as common-source outbreaks or point-source outbreaks. The former involves everyone who has been exposed to the same agent, such as the bubonic plague or the hepatitis A epidemic among patrons of a restaurant that served green onions. The latter involves a group of people who have been exposed to the same microorganism over a short period, such as the influenza epidemic of 2009 or the coronavirus (SARS-CoV) outbreak of 2020 that started in a single restaurant.
When a disease reaches epidemic proportions it is important to reduce the transmission rate as soon as possible to limit the number of cases and protect those who are still uninfected. The number of cases is plotted over time and a typical graph has a steep upslope followed by a gradual downslope (a log-normal distribution). To reduce the transmission rate measures such as quarantine, isolation, hand washing education and the use of airborne infection control equipment should be used.
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