Epidemiologist use prevalence and incidence as measures of how common a disease is.
Prevalence is the total number of individuals with the disease at a particular point in time divided by the population at risk. In other words, it is the proportion of the population that suffers from the disease.
Incidence is the number of NEW cases of the disease per year divided by the population at risk. For example there are approximately 150 new cases of osteosarcoma per year in Britain. The population of Britain is approximately 50,000,000. Therefore the Incidence of osteosarcoma is
150 / 50000000 or 3 per million.
So when the population and the disease are stable:
Prevalence = Prevalencestart Incidence Time
In trying to compare the influence of an exposure on a disease, it is obviously more useful to compare incidences rather than prevalences.