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Prevalence

Possible changes in prevalence over time.

In epidemiology , prevalence is a measure of the health of a population at a given moment. For a given condition, it is calculated by dividing the total population, the number of cases of disease present at a given time within a population (which the diagnosis was formerly worn or recently). Prevalence is a proportion which is usually expressed as a percentage.

Definitions

We talk about point prevalence if the measurement is made at a given time. This is the usual sense, which corresponds to the definition above.
We are talking prevalence over a given time (month, year ,...) when measuring the prevalence throughout the period rather than at a specific time period.
They talk about whole-life prevalence (in existence) when measuring the proportion of people who during their lives will be affected by a given pathology.

Relationship between prevalence and incidence a given time

Do not confuse prevalence over a given time and impact over the same period. Indeed, the annual incidence (or monthly) includes only new cases per year (per month), while the annual prevalence (or monthly) based on the total number of cases present, ie ie those already present over those incidents. Thus, the annual prevalence is still higher than the annual incidence.

If prevalence is low (P <5%), while the relationship between incidence and prevalence over the same period is P = I x D

Outbreak Status

In case of an outbreak there destabilization of the incidence, prevalence can then evolve very differently, such as the impact of disease on mortality.

If the disease has a very small influence on the mortality of the population at the end of the epidemic prevalence will decline slowly, the persistence of the disease (time required for the disappearance of its causative agent ) will be important and healing population will be long (figure A).

On the contrary, if the disease increases significantly the mortality rate, prevalence will decline more rapidly at the end of the epidemic, its persistence is shorter and recovery of the population is faster (figure B).