Health officials suspect the highly contagious Norovirus, which causes acute gastroenteritis, meaning an inflammation of the stomach and intestines. Stool samples are being collected and will be tested in the coming days.
Tough Mudder events involve lengthy "hard core" obstacle courses designed "to test all-around strength, stamina, mental grit and camaraderie," according to the Tough Mudder website.
But that means a lot of contact with bodily fluids, health officials said.
"You're crawling through mud, carrying logs, climbing over obstacles," said Angela Minicuci, spokeswoman for the Michigan Department of Community Health.
Virus-carrying saliva or fluid from one competitor can be easily transmitted to the next competitor touching the same surface, she said.
The symptoms so far have included cramping, diarrhea, vomiting - "all the things we'd associated with a gastrointestinal illness," Minicuci said.
Health officials have spoken to at least a dozen callers "and at least another dozen are coming in (from) throughout Michigan and in border states," said Lenawee County Health Officer Patricia Bourgeois.
The event at the speedway in Brooklyn, Mich., drew up to 20,000 people, though it's unclear how many were sickened. Most of the calls so far have been from competitors, but several spectators called as well, Bourgeois said.
"But we're not sure how that will unfold because we don't know the cause or what we're dealing with yet," she said.
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