The calculation for incidence is the number of new cases divided by the population risk. Remember in this question you were told that 500 of the 2500 have already tested positive for the disease. Now, you will have only 2000 at risk for the next year. Therefore, the annual incidence would be number of new cases divided by the population at risk which is 200/2000= 10%.

mm9283Is this not a silly question? It's assuming that those 500 individuals cannot get chalmydia again next year, but there is no reason they are still not at risk! It's not like they become immune to it +2020-05-17T04:49:41Z

submitted by aoa05(11), 2020-04-23T22:08:51Z

The calculation for incidence is the number of new cases divided by the population risk. Remember in this question you were told that 500 of the 2500 have already tested positive for the disease. Now, you will have only 2000 at risk for the next year. Therefore, the annual incidence would be number of new cases divided by the population at risk which is 200/2000= 10%.