Incidence and prevalence are used in expressing the frequency of a medical condition ((typically a disease or a risk factor), and should not be confused with each other.

Incidence is the rate of newly diagnosed cases of a disease, within a period of time (e.g., per year). It is generally reported as a fraction of the population affected (e.g., per 100,000 or per million population).
Prevalence is the total number of patients alive, either within a given period (= period prevalence, for example yearly prevalance) or at a particular date (point prevalence). Period prevalence is preferable to show the real frequency of the condition, because it includes all affected individuals (survivors and deaths) within a period, whereas point prevalence counts only those alive on a particular date.

Thus, incidence gives the risk of having the disease, whereas prevalence indicates how widespread the disease is. In other words, incidence answers "How many people per year newly acquire this disease?" and prevalence answers "How many people have this disease right now?"

The relationship between incidence and prevalence depends heavily on the clinical course of the disease. In a curable infectious disease with a short duration, the incidence may be high, but the prevalence is low. In a disease that has a low cure, but good survival rate, then incidence contributes to steady increase of prevalence.

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