Prevalence is the proportion of the population with the outcome of interest at a given point in time. It is often used to assess the burden of disease within a population.

Example:

In  2002, HIV tests were given to consenting women aged 25 or less, attending antenatal clinics in the Mwanza region of Tanzania.  Results were divided into four categories based on the location of the clinics: Rural – remote; Rural – roadside; Urban – town; and Urban – city.

In the Rural – roadside setting, 1042 women were tested. Of these, 107 were found to be HIV+.

From this we can calculate the prevalence:

Incidence quantifies the number of new events which occur in a population during a specified period of time.

Note that this means that any individual who already has the outcome of interest when the time period begins is not included. Only new cases count.

Example:

A group of female inmates at a prison in Sao Paulo, Brazil were tested for both HIV and tuberculosis (Ferreira et al. 1996). Of the women tested, 68 women who tested negative for tuberculosis were tested again 6 months later. At the second test, 21 of the initial 68 tested positive for tuberculosis.

From this information we can calculate the incidence of tuberculosis among these women over the six month period.