France's version of a Tough Mudder race resulted in a "mystery outbreak" because there may have been animal feces in the mud.
Nearly 8,400 people gathered at Nice's Côte d'Azur on June 20 for the Mud Day race. The 8-mile course contains obstacles such as electrified barbed wire and dunes covered in mud, Daily Mail reports.
But just one day after the race, 30 participants began experiencing diarrhea, vomiting and fevers, according to 9NEWS.
The days to follow saw more participants grow ill; over 1,000 people had these symptoms, but none of them required hospitalization.
A local health agency is currently investigating the cause of the illness.
Numerous participants reportedly smelled horse manure during the competition, suggesting the feces may have somehow gotten mixed up with the mud.
Health experts believe the feces emitted a bacteria similar to Salmonella that was ingested by participants, The Local reports. Many of the runners were entirely submerged in the mud at some points.
Event organizers posted a message on Mud Day's official site earlier today.
[The Mud Day organizers] have immediately decided to make every effort to determine what could have caused the stomach upsets.
According to RT, the post also noted everyone who took ill was soon cleared to go home.
Citations: More than 1000 runners fall ill after competing in French mud race where animal faeces had become mixed with the dirt (Daily Mail), More than 1000 runners fall ill after French mud race (9NEWS)