In
epidemiology, an
infection is said to be
endemic (from
Greek en- in or within +
demos people) in a
population when that infection is maintained in the population without the need for external inputs. For example,
chickenpox is endemic (steady state) in the
UK, but
malaria is not. Every year, there are a few cases of malaria acquired 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).
For an infection that relies on person-to-person transmission to be endemic, each person who becomes infected with the disease must pass it on to one other person on average. Assuming a completely susceptible population, that means that the
basic reproduction number (R<sub>0</sub>) of the infection must equal 1. In a population with some
immune individuals, the basic reproduction number multiplied by the proportion of
susceptible individuals in the population (
S) must be 1. This takes account of the
probability of each individual to whom the disease may be
transmitted actually being susceptible to it, effectively discounting the immune sector of the population.
For the disease to be in an
endemic steady state:
- <math>
times = </math>
In this way, the infection neither dies out nor does the number of infected people increase
exponentially but the infection is said to be in an endemic steady state. An infection that starts as an
epidemic will eventually either die out...
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