Why do cats get sick and when they get sick what are some symptoms of a couple sicknesses

Some cats immune system's are weak. Your cat can tell you if she is ill and her symptoms with her body language.

I dont really know the why enough to help out, but i know of some symptoms. k so cats like to hide it if they are sick so most people dont notice if there cat is sick. they will try to hide (more than normall). Some cats will bump there head against non animate object when they have something wrong too(not the sweet kind against ur leg or the one were they do it against objects to rub there back). Another one is vomiting, smelly pee, not eating, not going to the bathroom, eating there litter, extremly hungry(more than normal) or straining or meowing in the litter box. stuff like that. hope that helped :)