Participants in a recent Tough Mudder event have come down with rashes and infections, California authorities say.

“Anywhere on my body that touched the ground had red spots,” participant Chris Palakos told KTVU, adding that as many as 350 people who took part in the event have fallen ill.

“The Tough Mudder race involved extensive skin exposure to mud,” the Sonoma County health advisory said. “Most affected persons have pustular rash, fever, myalgias, and headache.”

Those symptoms may indicate a minor illness known as swimmer’s itch, but also could suggest a staph infection or a more serious bacterial infection such as Aeromonas, the advisory said.