The persistent conviction that one is never or is likely to never become ill, often involving blatant disregard for emergent symptoms, and persisting despite reassurance and medical evidence to the contrary.
It wasn't his hyperchondria that killed him, but listening to his doctor instead of people on a message board might have been a good idea.
Hyperchondria is the stubborn belief that one is healthy even when there is medical evidence to support the presence of an illness or symptoms indicating one has such an illness.
Tim's hyperchondria was so blatant, he would not even acknowledge that his gangrenous wound was infected. He said he was perfectly healthy instead.