Incidence and prevalence rates

Incidence and prevalence rates

The occurrence of disease can be measured by using incidence rates and prevalence rates. The incidence rate measures the occurrence of new cases of a disease in a population over a period of time. The incidence rate is an important measure for evaluating disease-control programs and has implications for the future problems of medical care. For example, the calculation of incidence rates of HIV/AIDS provides insight into whether the disease is spreading and whether HIV-prevention programs are working.
The prevalence rate measures the total number of existing cases of a disease in a population at a given point in time or over a period of time. The prevalence rate is a useful indicator of the burden of a disease on the medical and social systems of a geographic region. It is useful only for diseases of long duration (months or years). For example, within countries, prevalence rates can be used to determine the medical, economic, and social burden of AIDS.
Prevalence rates vary directly with both incidence and duration of disease. If the incidence of a disease is low but the duration of the disease is long, such as with chronic diseases, prevalence will be large in relation to incidence. Conversely, if the prevalence of a disease is low because of short duration (due to recovery, migration, or death), prevalence will be small in relation to incidence.