incidence rate

An incidence rate measures how quickly new cases of a disease arise. The top of the fraction is the number of people who newly develop the disease during a stated period. The bottom is the population at risk over that period, either the total time each person was watched for the disease, added up across everyone, or the average population during the interval.

Incidence counts only new cases. That is what separates it from prevalence, which counts everyone who has the disease at a given moment, whether they fell ill last week or years ago. A falling incidence rate therefore means fewer people are catching or developing the disease, not simply that existing patients are being cured.

Public health bodies usually express it per 100,000 people per year, so that countries and years of different sizes can be compared. The World Health Organization reports tuberculosis this way, as the number of people falling ill with TB per 100,000 population per year, and sets its End TB Strategy milestones as percentage cuts in that rate.