There are two relatively simple terms that get splattered about and are sometimes confused and can cause all sorts of difficulties.
Incidence – the number of people who develop a condition in a specific period of time.
(I think of this a bit like an ‘incident’ – a thing that happened.It probably has the same etymological root. Or something)
Prevalence – the number of people who have a condition at a specific period in time
(This, like a prevailing wind, is the weight of a disease burden at one point in history.)
But there is a touch more complexity …
If you want to know what proportion of children in special schools have visual difficulties at any one point in time, you’ll be asking for the prevalence of occular problems. If you want to be planning your print order for information packs from new diabetics, you’ll want the local incidence of DM.
Where conditions are not cured, and those affected are unlikely to die*, then the cumulative incidence is going to be, near as you like, the same as the prevalence.
* die within the time from developing the diagnosis and you wondering what proportion of folk have it. Not “be immortal” sort of “unlikely to die”.