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 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
_{0}) 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:
\ {R_0} \times {S} = {1}
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 (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 endemic steady state in a population, the
relation above allows us to estimate the
R_{0} (an important
parameter) of a particular infection. This
in turn can be fed into the
mathematical model of
an epidemic.
It should be noted that while it might be common to say that
AIDS is "endemic" in Africa, this is a use of
the word in its colloquial form (meaning found in an area). AIDS
cases in Africa are still increasing, so the disease is
not in an endemic steady state. It is more correct to call
the spread of AIDS in Africa an epidemic.
See also