Endemic

In epidemiology, a disease is endemic when it is always present in a particular population or geographic area at roughly the level health officials have come to expect. That usual amount is called the baseline or endemic level, and it is not necessarily low or acceptable. Malaria is endemic in many tropical regions, where it causes heavy illness year after year, and diseases that persist at especially high levels are called hyperendemic.

The word sits on a scale with related terms. A sporadic disease appears only now and then, without a regular pattern. An epidemic is a rise above the endemic level, and a pandemic is an epidemic that spreads across several countries or continents. For an infection, the endemic state is a rough balance in which each case leads, on average, to about one new case, sustained by a steady supply of susceptible people through births or waning immunity, or by a constant animal or insect reservoir.

Knowing the endemic level lets surveillance teams spot outbreaks, because only a clear rise above it signals an epidemic. In ecology the word has a different sense: an endemic species is one found naturally in a single place and nowhere else, such as the lemurs of Madagascar.