A disease in a population that is spreading faster than the normal rate of disease. This is in contrast to endemic diseases, which are always present at a low level in the community and require a certain amount of effort to control (see Definitions). Examples include cholera in Yemen, measles in Burundi, Ebola in DRC and Congo, SARS-CoV-2 in Asia, and dengue fever worldwide. Typically, epidemics can be contained with a combination of medical and non-medical strategies such as treatment, isolation, quarantine and vaccination.
The word epidemic derives from the Greek epi- meaning “upon” and demos, meaning people. Hippocrates, who wrote the Corpus Hippocraticum around 400 BC, used the term epidemios to describe groupings of symptoms and illnesses, with reference to atmospheric characteristics or seasons, and sometimes describing propagation of a particular syndrome in a human population (“on the people”).
An epidemic is usually caused by direct contact between humans, such as through some sexually transmitted diseases; through sharing an item that passes the disease, such as needles; or through a vector like mosquitoes. In the case of the 2014 outbreak of Ebola hemorrhagic fever in western Africa, it was spread through contact with body fluids as families tended to sick relatives and cared for them.
If an epidemic is large enough to affect a significant portion of the global population, it is considered a pandemic disease (WHO, 2020b). Other factors that define a pandemic are the length of time over which cases increase (the duration of the “epidemic curve”), whether the number of cases decreases again before increasing again (“decline and rebound”); the percentage of individuals who become infected but do not die; and how rapidly the number of cases increases at a given point in time.