State officials say participants and spectators at an endurance event last weekend at Michigan International Speedway were exposed to Norovirus.
The Michigan Department of Community health says the outbreak at the "Tough Mudder" contest has been confirmed.
A spokeswoman says more than 200 reports of gastrointestinal symptoms were reported since July 1.
She says in most people, the illness lasts for one to two days and includes nausea, vomiting, diarrhea and abdominal cramps.
But people who contract norovirus should stay home for up to 72 hours after their symptoms go away.
There's no medication to treat norovirus, but health officials urge ill people to drink plenty of fluids to prevent dehydration.
The MDCH says the illness is transmitted through touching virus-contaminated surfaces and objects, eating or drinking contaminated food, or by direct person-to-person contact. The agency says it is highly transmittable and hand sanitizers are not effective against the virus. Frequent hand washing with warm, soapy water for 20 seconds is crucial to preventing the spread of the disease.
There have been 119 suspected outbreaks of norovirus in Michigan so far this year.