That's the thing about Sean--his physical body and such is strong, but his internal systems are really screwy...so things that would normally just mess up a regular person for a while would either kill him or mess him up permanently. Mostly drug-related, including prescription drugs... but despite that, he has a fairly powerful immune system, so he never gets sick anyway.

Yes, there's a reason for this. It's not a particularly original reason, but it's still a reason.