The Michigan Department of Community Health and the Lenawee Counth Health Department confirmed that numerous people were indeed sickened with the norovirus at the Tough Mudder games in Brooklyn on June 29-30.
Most of the ill caught the virus on the race course itself, generally from getting mud or water in a participant's mouth.
Racers went in waves, with the first outbreaks happening in the early heats on Saturday.
The number of people infected increased with successive heats thoughtout the weekend.
Health officials suspect one racer in the early heats had the norovirus.
Everyone who was infected should be healthy by now.
The Tough Mudder association is now working with the health departments to find ways to avoid a outbreaks at future events.