After France’s Mud Day race at Nice’s Côte d’Azur, which took place on June 20, 1,000 of the 8,400 mud run participants reported symptoms of acute gastroenteritis, including diarrhea, vomiting and fever. The regional health agency says that these symptoms are caused by the norovirus, a highly infectious disease caused by the ingestion of contaminated food.

But, doctors say that they have diagnosed some people who did not eat anything at the race. Many runners claim that sections of the course smelled distinctly of horse manure. It is likely that runners accidentally ingested muddy water on the mud run course that was contaminated by feces.

Thirty people reported feeling ill the day after the race, with that number climbing to 1,000 in the following days. Race organizers have issued a statement that they are making every effort to get to the bottom of the cause of the “stomach upsets.”