Fig. 11. Onset and resolution of a hypothetical ocular infection in an open population (o, onset of infection; 'resolution of infection). The point prevalence of this disease at December 31 is the number of existing cases (i.e., cases 2, 3, 6, and 8) divided by the estimated population at December 31. The incidence or attack rate for 1 year is the number of new cases (i.e., cases 3, 4, 5, 6, and 8) divided by the population at July 1.