Prevalence is how common something is — how much of it there is in a place, at a time, or among a group of people. It is a noun: it names the fact, not the thing itself. Doctors and news writers use it a lot. They might talk about the prevalence of a sickness, meaning how many people have it now. That is not the same as how many new people catch it this week; that other number has a different name. You can also talk about the prevalence of a habit or an idea, though that sounds rather formal. Everyday English often just says how common it is, or how widespread. The word does not tell you if the thing is good or bad — only how much of it is around.