Disease prevalence and incidence are concepts that provide information on how common a disease or disorder is within a population, and how frequently it occurs. In this article we explain how to calculate disease prevalence and incidence. 

Prevalence

The prevalence of a disease (or condition) refers to the number of people who have that disease expressed as a proportion of the population. Generally, prevalence is expressed as a fraction, percentage or the number of cases per 10.000 or 100.000 people in a population. There are 3 subtypes of prevalence:
(1). Point prevalence: The proportion of a population that has the disease/condition at one specific point in time.
(2). Period prevalence: The proportion of a population that has the disease/condition at some point during a given period.
(3). Lifetime prevalence: The proportion of a population that at some point during their life have experienced the disease/condition. 

The general formula to calculate the prevalence of a disease within a population is:
Prevalence

Incidence

The incidence of a disease refers to the number of new cases within a population in a specified time period. We can distinguish between 2 subtypes of incidence:

(1). Cumulative incidence (also called incidence proportion): The number of new cases within a specified time period divided by the size of the population initially at risk. When assessing cumulative incidence, one prerequisite is that we are looking at a closed population, meaning that the population can only decrease (due to death, loss of follow-up etc.) and that subjects entering the population during the evaluation period are not included in the evaluation. Furthermore, all members of a population have to be ‘at risk’ at T = 0 (time), so we exclude all prevalent cases at T = 0 (people that already have the disease at T = 0 are excluded in incidence counts). We can calculate cumulative incidence using the following formula:
Cumulative incidence
Using this formula, incidence is expressed as the number of new cases per 100 people in the population in time period P. Cumulative incidence is also frequently expressed as the number of new cases per 1.000, 10.000 or even 100.000 people in the population in time period P.

Example of cumulative incidence : At T = 0 a population has 1000 disease-free persons, and during a two year evaluation it turns out that 34 people contract the disease that is being investigated. The cumulative incidence is then 34 cases per 1000 persons per two years, i.e. 3.4% of the population contract the disease in this time period. However, for some diseases it is possible that the same person contracts the disease multiple times during the timespan of the study (e.g. the flu). The same person getting sick twice, does often count as multiple incidents of the disease within the population. This could mean that an incidence of 34 cases per 1000 persons only affects for instance 2.8 % of the population, since a substantial number of people got sick multiple times due to a weakened immune system. Therefore, it might be more correct to express incidence as the number of new cases per 100 (or 1000, 10000 etc.) instead of expressing it as a percentage of the population.

(2). Incidence density: With incidence density we include the phenomenon ‘person-time’ in the equation. Person-time is the number of persons and their time contribution in a study. We look at how long someone has been ‘at risk’ during the timespan of a study. For example, if during a 1 year study, a person contracts a disease after 45 days, their person-time is only 45 days. Another individual that stayed healthy during the entire study, has a person-time of 365 days. Combined, these individuals contributed to 410 days of person days at risk (person-time). Person-time is frequently expressed in years, but it can also be expressed in months for instance. Incidence density can be useful when dealing with a dynamic population, allowing you to include additional participants during the study. Incidence density is calculated with the following formula:
Incidence density

Example of incidence density: We follow a dynamic population over a 6 year period. During this 6 year period, additional participants enter our population and are included in our study. The following figure shows when participants enter or leave the study, and for each participant the person-time is calculated on the right side. As can be seen in the yellow box, the incidence density is 0.09 new cases per year of person time.
Example incidence density