An outbreak of disease that spreads from person to person, as opposed to a disease that originates in animals and then spreads to people (like typhoid or HIV). An epidemic is distinguished by the rapid increase in cases. Examples of epidemics include cholera, smallpox and yellow fever.
The word epidemic comes from the Greek epi- meaning upon or above and demos-meaning people. Hippocrates used the term in 430 BC to describe seasonal outbreaks of diseases such as cholera, malaria and tuberculosis. Over two thousand years later, the word epidemic has evolved to refer to specific diseases, but its underlying meaning has remained unchanged.
Some epidemics are mixed, meaning they have characteristics of both common-source and propagated outbreaks. In these, the first people infected become the source of the next wave of infections, as with the 1980s Lyme disease epidemic caused by tick bites. Others are propagated, where the disease spreads from person to person like a wildfire. The COVID-19 epidemic is an example of this, as has been the recent swine flu pandemic. In both cases, the speed and scope of the epidemics has exceeded all previous records.