I've been relatively lucky

I had to have my appendix out when I was about 14, they had to swill my guts out because it was so close to bursting that the poison had started to seep out of it.

The worst thing I've ever experienced though was shortly after getting the flu I got an infection on the lining of my skull (or brain I can't remember now)
Completely bed ridden for well over a week, the one time I did try to go downstairs it took me over 45 minutes to get from my bedroom to my living room.
Every time I moved I had this searing pain which at times took away my vision it was that bad.

I was in my early twenties then, probably should have gone to the hospital about that but my parents obviously didn't think it was all that bad I just went to the doctors after a week had passed.