On October 12, 2012, the Nellis Air Force Base Public Health Flight (Nellis Public Health), near Las Vegas, Nevada, was notified by the Mike O'Callaghan Federal Medical Center (MOFMC) emergency department (ED) of three active-duty military patients who went to the ED during October 10-12 with fever, vomiting, and hemorrhagic diarrhea. Initial interviews by clinical staff members indicated that all three patients had participated October 6-7 in a long-distance obstacle adventure race on a cattle ranch in Beatty, Nevada, in which competitors frequently fell face first into mud or had their heads submerged in surface water. An investigation by Nellis Public Health, coordinated with local and state health officials, identified 22 cases (18 probable and four confirmed) of Campylobacter coli infection among active-duty service members and civilians. A case-control study using data provided by patients and healthy persons who also had participated in the race showed a statistically significant association between inadvertent swallowing of muddy surface water during the race and Campylobacter infection (odds ratio = 19.4; p<0.001). Public health agencies and adventure race organizers should consider informing race attendees of the hazards of inadvertent ingestion of surface water.