**Prevalence**

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**

**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 = Prevalence**_{start} 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.