Do kittens get sick even if they are properly cared?

I have a 3 1/2 months old domestic breed kitten and I'm taking care of him properly. It means that he is being vaccinated, getting good quality food and his litter box is always clean. He has never been out of home except for vaccination. He is gaining weight and physically active. Is there a chance he'll get sick after so much care? If yes, what sort of sickness he can get into?