Late Friday afternoon, the Michigan Department of Community Health who participated in last week's Tough Mudder event. And calls reporting the illness are continuing to come in.
About 16,000 people participated in the endurance contest at the Michigan International Speedway.
About 100 of them have called the Lenawee County or state health departments to report Norovirus symptoms and eye infections.
About 1,000 participants, spectators and workers are being asked to complete a survey from the state health department.
"We're working to get a survey out to participants to see if we can determine the source of the illness. That's the most important thing at this point." said Angela Minicuci of the Michigan Health Department.
The health department warns that Norovirus it is highly contagious and it can be spread to friends and family.
It is recommending that people who think they're sick take precautions like washing hands and avoiding contact with other people until symptoms have passed.