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In epidemiology, an infection is said to be endemic in a specific population or populated place when that infection is constantly present, or maintained at a baseline level. The term describes the distribution of an infectious disease among a group of people or animals or within a populated area. An endemic disease always has a steady, predictable number of people or animals getting sick, but that number can be high (hyperendemic) or low (hypoendemic), and the disease can be severe or mild. Also, a disease that is usually endemic can become epidemic.
If a disease is in an endemic steady state in a population, the relation above allows the basic reproduction number (R<sub>0</sub>) of a particular infection to be estimated. This in turn can be fed into a mathematical model for the epidemic. Based on the reproduction number, we can define the epidemic waves, such as the first wave, second wave, etc. for COVID-19 in different regions and countries.
Misuse
It has been claimed that endemic COVID-19 implies that the disease severity would be mild. but in itself endemicity only means that there will be a steady, predictable number of sick people.
;Hyperendemic: An endemic disease with a high rate of infection, This term is often used to describe the prevalence of malaria in a local area, with 10 to 50% of children showing evidence of prior infection being considered a moderate level for that disease.
;Hypoendemic: An endemic disease with a low rate of infection.
Categories for non-endemic diseases
;Sporadic: A disease that appears occasionally, but, unlike endemic disease, is not always present at a steady and predictable level.
;Outbreak: An epidemic, especially one affecting a very small area, such as the people in one town or attending a single event.
- Lassa fever