That's probably what they have done Nindigo. There was just some confusion when the dev on the Vet Clinic live stream said a few times that pets will get sick every two to three sim days. This has been changed/corrected to: pets will get sick maybe once or twice in their life span. However there are vet treats your sims can make to err... make your pet sick should you want them pooping gold bars (which your sim can sell- yes really, this is a thing), or just to experience taking them to an NPC vet etc etc.
I think the sickness side of pets should be fairly well balanced if they are only getting sick by eating bad food, one or twice by RNG game mechanics or because your sim is feeding them 'negative' treats.