We explain what incidence is, its differences with prevalence and what the incidence rate is. Also, what is an incident.

In medicine, incidence is the number of new cases of a disease.

What is incidence?

By the term incidence, we generally refer to a event or happening that occurs in the middle of a subject and that he has some relation to it. This general meaning comes from the very origin of the word, which dates from the Latin incidentcomposed by inside- (“into”) and the verb chair (“fall”), so that figuratively it is understood as something that “falls” or happens within the frame (“inside”) of a subject.

In this sense, an incident is understood as something fortuitous, sudden, unexpected, which occurs in the midst of a subject or event. That is why we use the expression “by incidence” as a synonym for “by chance”, and the verb influence as a synonym for “to affect”, that is, to have an unpredictable effect on something (“the rains affected the climate”).

However, one of the most common uses of incidence is the one that has to do with medical terminology, that is, with the world of health and medicine, and that generally refers to the number of new cases of a disease occurring within a population determined within a certain period of time. That is, how many infections occurred in a population within a period of time.

The incidence of a disease can also address specific conditions, for example:

  • recidivism: The reappearance of the disease in patients who already suffered from it before.
  • Cumulative incidence: The percentage probability that a healthy individual will develop the disease within a given period of time.

This information is always useful in designing public health policies or in combating a particularly contagious disease in the context of an epidemic.

It can help you: Prevention

Incidence rate

The incidence rate or incidence density is, in the context of the study of a disease, the statistical relationship between the number of new cases in a given period of time and the sum of all observation periods. This means that it is an indicator expressed in cases-time, to express how often a disease appears in a given population over time.

The incidence rate is calculated by dividing the total number of new cases by the sum of the time periods. Let’s look at some examples of this:

  • In the midst of a study on flu recurrence, the health of 10 people of the same age group is monitored for two years. Of these, 6 contracted the flu on more than one occasion. Therefore, the recidivism rate reported in the study will be 6/2, ie 3 person-years.
  • Another study focuses on the occurrence of a particular sexually transmitted disease in adult men, studying a population of 5 volunteers for up to five years. Among them, two were studied for all five years, another two for four years and the rest for only one. At the end of the study, the disease had occurred in only 2 of the subjects studied. Thus, the incidence rate would be 2/(5+5+4+4+1) = 2/19 = 0.10526 person-years.

Prevalence and incidence

Prevalence and incidence are different but similar statistical factors. Prevalence refers to the persistence of a disease over a period of time.; incidence, on the other hand, always refers to new cases.

In other words, prevalence refers to the ratio: how many affected individuals there are in relation to the total population. Therefore, it is expressed as a fraction, a percentage or a specific number of cases per 10,000 or 100,000 population.

Unlike incidence, which expresses the rate of disease onset (hence the measurement time), prevalence can be understood as how common the disease is in the population, that is, what percentage of the population receives. The higher the prevalence, the more common the disease; the lower the prevalence, the rarer it is.

Continue with: Morbidity


  • “Incidence” on Wikipedia.
  • “Incidence” in the Language Dictionary of the Royal Spanish Academy.
  • “Incidence” at the Ramón y Cajal University Hospital (Spain).
  • “Incidence rate” in the verification of the Government of Spain.
  • “Arquivo de Incidência” in the Online Spanish Etymological Dictionary.