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Incidence is the rate of new cases or events per unit time in the population at risk for the event. In medicine, incidence is commonly defined as the newly identified cases of a disease or condition per population at risk over a specified timeframe.[1] An example of incidence would be 795,000 new strokes in the United States annually. Here, the incidence is 795,000 new strokes per year, the population in the United States is 325 million, and the timeframe is 1 year. Alternatively, incidence can be specified as person-years. For example, there may be 324 million people in the United States for the measured year, so strokes could be specified as having an incidence of 2.5 strokes per 1,000 person-years. This means there are, on average, 2.5 strokes per 1,000 people in the United States over 1 year. To calculate the person-years incidence of strokes in the United States, we perform the following: (795,000 strokes)/(324,000,000 people in the United States during the year) = 2.5 strokes / 1,000 person-years.

Incidence is key to understanding for the healthcare team. Many aspects of medicine depend on allocating resources based on either the incidence or prevalence of a disease process. The health care team, as a whole, must thus concurrently decide whether the case is a new case (incident case). Additionally, incidence has significant implications for public health and epidemiology, as many outbreaks are identified and then tracked through incidence to help quell the spread of the disease.