In epidemiology, the prevalence is a measurement of the health condition of a population at a given moment. For a given affection, it is calculated by reporting to the total population, the number of cases of Maladie S present at one moment given in a population (that the diagnosis was carried in the past or recently). The prevalence is a proportion which is expressed as a percentage generally.
One should not confuse prevalence and incidence .
The Incidence holds account per annum only new cases. Whereas the Prévalence is based on the full number of case present, i.e. those already present plus those incidental. Thus the prevalence is always higher than the incidence.
The relation between Incidence and Prevalence is P = I X D with P=La Prévalence, I=L' Incidence, D=La Intermediate duration of the disease
This relation is valid if the prevalence is weak (P<5%).
Additionnellement, in the event of epidemic there is destabilization of the incidence and the prevalence can evolve/move very differently, for example according to the impact of the disease on the Death rate. If the disease has a very weak influence on the death rate of the population, at the end of the epidemic the prevalence will decline slowly, the persistence of the disease (time necessary to the disappearance of its agent etiologic) will be important and the cure of the population will be long (diagram has ). On the contrary, if the disease increases in an important way death rate, the prevalence will decline more quickly as of the end of the epidemic, its persistence will be less long and the cure of the population will be faster (diagram B ).
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