Prevalence - indicates the proportion of people in a population who have a certain sign or disease at the time point of the research, irrespective of when they fell ill or when the signs appeared. In order to be able to compare prevalence in populations, we must also know the population where the proportion of the people was measured in addition to the proportion of the people itself, as well as the time point of the measurement. By measuring prevalence we can follow a decrease of a certain disease due to improved preventive measures (e.g. prevalence of HIV infection) as well as determine the incidence of a certain disease on a lifetime basis, in a certain life period or annually.