I'm afraid I haven't figured out exactly what causes a pet to get sick. I have only had my T5 for less than a week, and haven't been playing with my pet that long. Sorry. Maybe someone else has an idea? I do know that when I gave my sick pet two doses of medicine about 20-30 minutes apart, he was dead the next time I checked on him - it wasn't a pretty sight! Maybe your pet is sick now from an overdose of medicine? I hope not, because, if he is, you'll be adopting a new pet soon.