Example:
In 2006, 4 new cases of an incurable viral disease were diagnosed in a population with 100 individuals. That brought the total number of affected individuals in the population to 16.

The incidence of this disease in this population in 2006 was 4/(100-16). The number at risk was 84 since 16 were already affected.

The prevalence of this disease in this population in 2006 was 16/100.