Incidence measures the number of new cases of a disease or condition developing in a population over a specific period, reflecting the risk of contracting the illness.
Prevalence measures the total number of existing cases (both new and pre-existing) at a particular point in time, reflecting the overall burden of the disease.
The two metrics are fundamentally connected by the duration of the disease.
The mathematical relationship explains why a condition can have a high prevalence but a low incidence.
For example, chronic conditions like diabetes have low incidence rates (fewer people get it) but very high prevalence because patients live with the disease for a long time.
Conversely, a highly contagious acute illness, like the common cold, has a high incidence but low prevalence, as people get sick quickly but recover just as fast.