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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