I have seen situations where these two words, very common in epidemiology, are used interchangeably. To tell the truth I may have done that as well.
The fact is that prevalence measures the cases existing in a given population whereas incidence measures only the new cases, it is the rate to which new cases happen.
The prevalence is often measured as a percent or per 100.000. In epidemiological studies, it usually is measured by a question asking whether or not the respondent has the condition and the estimate is the number of positive answers divided by the total number of interviewed. In this case the prevalence is valid for the target population covered by the survey. It is also valid for that point in time, it is a photography of the population on that point in time, as some like to say.
On the other hand the incidence is a rate and therefore it has a time period as part of the measure. For example, one could say that the incidence of disease X is 1% per year in a given population, meaning that in a year, 1% of the population without the condition will acquire the condition. In a survey one would ask respondents whether or not they acquire the condition over the course of the past 12 months and the positive answers would be the numerator in the incidence estimation. The denominator would be these same respondents plus those who were disease free in the past 12 months.
When the number of cases is known for more than one year, it is usual to report the incidence in terms of person-year. For example, 20 cases in 1000 people in two years can be reported as 2% incidence per 2 years or 1% incidence per person-year, dividing the incidence by the number of years. However, if the incidence is not constant over years then this type of reporting is not a good idea since it just show the average across two years where incidences on each year are quite different.
Therefore these two measures can be quite different things and should be reported according to their definition to avoid misinterpretation or results.
