It&#x27;s probabilistic because <i>you don&#x27;t know</i> whether or not you have an illness. It&#x27;s like tossing a coin - it&#x27;s 100% on one side and 0% on the other, but you don&#x27;t know which side it is on until you check - that&#x27;s why we say a fair coin has a 50% probability of landing on either side when tossed.<p>Now the test you use with that (mathematical) coin is 100% accurate. Tests in medicine are not. They&#x27;re more like &quot;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&quot;. Hence the uncertainty.