Definition: The prevalence of a disease is the number of people in a population who have that disease at a specific time divided by the total population.
Example: The prevalence of HIV infection in several African countries is above 20 per 100 population.
Additional information: Prevalence is the product of incidence and duration. Most infectious diseases have a rapid course such that prevalence is not very interesting. But in chronic infections prevalence gives some indication of the risk of exposure to others in the population.