My 3 year old daughter often asks that too. She also asks why do I always need shots? (referring to the bloodwork), and why does the dr. always look at my vagina? And why do I need a shot when I throw-up? For now we say "to help you grow just right, to make sure you have the right kind of medicine, and because you had a boo-boo there", in that order, and she accepts this. We will always try to be honest in a way she can understand. All children are different, so you'll find what works for you. We haven't really used any words yet like disorder, condition, sickness or disease. Maybe later we'll use condition or something similar.