This is a measure of the total number of infected or diseased individuals at a specific point in time. For example, the U.S. Dept. of Agriculture found that the apparent prevalence of MAP infected U.S. dairy herds is 60% (observed rate of test-positive dairy herds). Prevalence can be either the true, actual, infection or disease rate, or it can be the apparent prevalence, often called the test prevalence, meaning percentage of a population that tests positive. True prevalence can only be estimated from the test prevalence if you know the performance characteristics (sensitivity and specificity) of the test. The estimated TRUE prevalence of MAP infected dairy herds in the U.S. is 91%.