[TeMPOraL]

It's probabilistic because you don't know whether or not you have an illness. It's like tossing a coin - it's 100% on one side and 0% on the other, but you don't know which side it is on until you check - that's why we say a fair coin has a 50% probability of landing on either side when tossed.

Now the test you use with that (mathematical) coin is 100% accurate. Tests in medicine are not. They're more like "oh I see you sort of seem to have X; X has been known to occur a bit more in people suffering from Y than in those not suffering from Y". Hence the uncertainty.