In epidemiology, incidence is a measure of the probability of occurrence of a given medical condition in a population within a specified period of time. Although sometimes loosely expressed simply as the number of new cases during some time period, it is better expressed as a proportion or a rate with a denominator.
The number of registered cases of whooping cough in the United States has increased significantly. Especially children under the age of 5 months are at the most risk, since they haven't been fully vaccinated against the disease.
The number of cases was 19,798 in the '90s, an increase of 49%...