I did a 52 miler with a bladder infection. Terrible idea but it was early on and I didn't realize I had one right away. Did I mention there is only one place for a bathroom on the ride? It was not a good time and I wouldn't wish it on anyone.

I also rode a century with what I believed to be allergies. Turned out to be a really bad sinus infection and I was out sick from work the next two days. I rarely miss work.