Prevalence

The easiest way to describe diseases in populations is to just count the number of cases.

But this information is more useful if it's given in the context of a whole defined population.

Then you can see the proportion of people who have the disease at any one time.

This is called "prevalence" and can be expressed symbolically: a/a+b.

This is a proportion. A ratio would not include 'a' in the denominator: a/b