**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.