I came down with Cecal Vorvulus, a twisted intestine. At it's worst on the 19th of February I was in the hospital screaming in pain because the narcotics weren't doing enough. I ended up having about a foot of intestine removed, ultimately sidelining me for 6 weeks.
The theory is it was caused by a colon that had looser connective tissue than most people, coupled with a large amount of running. This caused the colon around the cecal valve to stretch and twist, eventually pinching off. The only cure was surgery to remove 5 cm of small intestine, the cecal valve, and the vertical section of the large intestine. Recovery is generally 6-8 weeks, and I was given the ok to jog at 4 and all clear at 6.