staph infection
from crashing onto a broken bottle embedded in the earth on a downhill course at the Malverns.

or glandular fever when I were a teenager.

But otherwise I'm an impervious wall of good health, fuelled by meat, exercise, booze and fags.

EDIT - although I did put myself into anaphylactic shock a few years ago after an "incident" in one of my labs and ended up in intensive care with half a litre of IV antihistamine and steroids in one go. that was fun. Still, happened about lunchtime and I was in the pub by 4pm. Happy days.
( the mighty badger Aphrodite, on a bar stool, by your side, Wed 17 Jun 2009, 12:27, archived)

I had something similar where whenever a wide faced terrier would bark at me;
I had a Staff Infection.
( Donkey Gums @mattcomedy, Wed 17 Jun 2009, 12:31, archived)

arf.
and also woof.
( the mighty badger Aphrodite, on a bar stool, by your side, Wed 17 Jun 2009, 12:34, archived)