Various diseases. We're immune to high levels of radiation, but things like the measles, flu, the chickenpox, and hepatitis weren't around on the Ark so we're easily susceptible to getting them. The vaccines will prevent you from catching them, and believe me, you don't want to.
You're not supposed to ask, but you'd figure it out eventually if I didn't. I turned thirty a few months ago and you about burned down my apartment trying to light the cake.