Definition: Incidence rate is defined as the number of individuals who fall ill with a certain disease during a defined period, divided by the total population.

It is used to give the probability of developing a particular disease during a given period of time; the numerator is the number of new cases during the specified time period and the denominator is the population at risk during the period.

If the time period is not stated it is assumed to be one year. Incidence rate may be expressed as a percentage or as per 1000 population.

Example: The incidence of Hepatitis B in Sweden is about 2 per 100,000 which means that in Sweden, some two individuals contract Hepatits B each year.