An epidemic refers to an increase, often sudden, in the number of cases of a disease above what is normally expected in a population within a defined area【478164031150476†L59-L66】. It is a sudden disease outbreak that affects a large number of people in a particular region or community【484534305866744†L14-L41】.
Explanation
Epidemics are identified relative to the baseline or endemic level of disease. When the incidence of a condition rises beyond what is expected for that area, epidemiologists declare an epidemic【478164031150476†L59-L66】. The Centers for Disease Control and Prevention note that epidemics can involve infectious diseases like measles or influenza or non infectious phenomena such as obesity or smoking when rates exceed the normal occurrence【798340332279714†L156-L166】. Epidemics occur when a pathogen and susceptible hosts are present in sufficient numbers and effective transmission can occur【478164031150476†L69-L80】. Factors that can trigger an epidemic include increased virulence of the agent, introduction of the agent into a new setting, more efficient transmission modes, changes in host susceptibility, or increased exposure【478164031150476†L69-L80】. Outbreak and cluster are terms often used for localized epidemics, whereas a pandemic is an epidemic that crosses international boundaries【478164031150476†L59-L66】. Epidemiologists classify epidemics as common‑source (point, continuous or intermittent) or propagated depending on how cases spread through the population.
Historical examples and characteristics
Notable epidemics illustrate the diversity of causes and transmission patterns. The 2014 Ebola epidemic in West Africa was a propagated outbreak in which the virus spread through contact with contaminated body fluids【484534305866744†L52-L60】. In contrast, the 1854 cholera outbreak in London was a continuous common‑source epidemic traced to a contaminated water pump【484534305866744†L63-L75】. Foodborne outbreaks, such as mass food poisoning at an event, represent point‑source common‑source epidemics【484534305866744†L63-L68】. Epidemics are not limited to infectious agents; a rapid increase in obesity rates or West Nile fever in a region are considered epidemics【798340332279714†L156-L166】. As epidemics evolve, they may develop into pandemics if transmission spreads across countries; however, most epidemics remain geographically contained.
Epidemics arise when disease incidence exceeds expectations and may involve a range of agents and transmission patterns. Recognizing the factors that precipitate epidemics and understanding their dynamics is essential for public health responses and for distinguishing epidemics from endemic disease or pandemics.
Related Terms: Endemic, Pandemic, Outbreak, Epidemiology, Epidemic curve