Incidence and Prevalence
Incidence is how many new cases arise; prevalence is how many people have it at one time.
What it means
Incidence — how many new cases occur in a population over a period. "50,000 new diagnoses a year."
Prevalence — how many people have the condition at a given moment, new and old together. "Two million people are living with it."
They come apart in a way that catches people out. A condition people live with for decades has a high prevalence and may have a modest incidence. A condition that resolves quickly, or is quickly fatal, can have high incidence and low prevalence.
Rising prevalence often means better survival rather than more disease, which is the opposite of how it reads in a headline.
In practice
In news reporting, charity campaigns and public health statistics.
Often used interchangeably by people who should know better, which is where much of the confusion comes from.
Why it matters
Check which one a figure is before drawing a conclusion. "Cases have doubled" means very different things depending on which is meant.
Ask whether a rise reflects more disease, better detection, or people living longer with it. All three produce a rising line on the same graph.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026