What is the difference between incidence and prevalence in epidemiology?
Question
The other day in class, someone asked about the difference between incidence and prevalence, and it got me thinking. I had read about this while prepping for an exam. Incidence tracks new cases over time, while prevalence shows how common a condition is at one point. This guide lays out the difference clearly with examples and practical uses.
Answer ( 1 )
Incidence
Incidence counts new cases that appear in a population during a defined time window, for example the number of fresh influenza infections per 1 000 persons in January. It measures risk of becoming a case.
Prevalence
Prevalence looks at all existing cases, old and new, at one moment or across a period. It answers “how widespread is the disease right now”. Chronic diseases often have low incidence but high prevalence.
Units and math
Incidence is expressed as a rate, so you divide new cases by person‑time or by the population size and add the time frame in the denominator. Prevalence is a proportion, cases divided by population at that moment, no time component in the denominator.
When to use which
Use incidence to study causes and to evaluate vaccines because it reflects the speed of transmission. Use prevalence to plan hospital beds or drug stocks because it reflects burden on the health system.
Quick example
If 50 students in a college of 5 000 catch norovirus in one week, the weekly incidence is 10 per 1 000. If on Friday 70 students are still sick, the point prevalence that day is 14 per 1 000.