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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, without extra infections being brought into the group as a result of travel or similar means. The term describes the distribution of an infectious disease among a group of people or animals or within a populated area. (2022). 9781529785814, SAGE. ISBN 9781529785814 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.
For example, chickenpox is endemic in the United Kingdom, but malaria is not. Every year, there are a few cases of malaria reported in the UK, but these do not lead to sustained transmission in the population due to the lack of a suitable vector (mosquitoes of the genus Anopheles). Consequently, there is no constant baseline level of malaria infection in the UK, and the disease is not endemic. However, the number of people who get chickenpox in the UK varies little from year to year, so chickenpox is considered endemic in the UK.
In this way, the infection neither dies out, nor does the number of infected people increase exponentially. An infection that starts as an epidemic will eventually either die out (with the possibility of it resurging in a theoretically predictable cyclical manner) or reach the endemic steady state, depending on a number of factors, including the virulence of the disease and its mode of transmission.
If a disease is in an endemic steady state in a population, the relation above allows the basic reproduction number (R0) 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.
Smallpox was an endemic disease until it was eradicated through vaccination.
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