Late this afternoon, the state health department confirmed that Norovirus is what is sickening participants in last week's Tough Mudder Event.
And more complaints of sickness are still coming.
About 16,000 people participated in the endurance contest at Michigan International Speedway.
Thus far about 100 of them have called the Lenawee County or state health departments to report Norovirus symptoms and eye infections.
The state health department is asking 1,000 participants, spectators and workers to complete a survey.
"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 is highly contagious and can easily spread to friends and family.
It recommends people who think they're sick take precautions like washing hands and avoiding contact with other people until symptoms have passed.