In epidemiology, an infection is said to be endemic in a population when that infection is constantly maintained at a baseline level in a geographic area without external inputs. For example, chickenpox is endemic (steady state) in the United Kingdom, but malaria is not… While it might be common to say that AIDS is “endemic” in many African countries, meaning found in an area, this is a use of the word in its etymological, rather than epidemiological, form. AIDS cases in Africa are increasing, so the disease is not in an endemic steady state. The spread of AIDS in Africa could correctly be called an epidemic, however.
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