Epidemiologists use measures of disease frequency, such as prevalence and incidence, to understand and describe a disease concerning the population size within a given time frame.
Within measures of disease frequency, prevalence looks at the number of confirmed cases of the disease within a given population. This measure is specified to a time, called point prevalence, or over a period of time called period prevalence.
In comparison, incidence refers to the number of new cases of a disease being encountered within a given population. The time for the incidence measures is also specified. Incident rates are the frequency of new cases occurring in the population.
Incidence risk refers to the healthy individuals in the population at risk of developing the disease at a given time. At the start of the spread, the whole population is considered at incidence risk, but with time, these numbers decrease.