The 1.5 million people who take part in obstacle races every year are ready for bumps, scrapes and bruises as they crawl through mud bogs, climb walls and wade through standing water. But no one signs up for a Tough Mudder, Warrior Dash or Spartan Race expecting to spend the next week plagued by fever, vomiting and bloody diarrhea.

That's what happened to 22 people who took part in a long-distance adventure race held on a cattle ranch in Beatty, Nev. in 2012, almost certainly because they swallowed muddy water contaminated by animal feces, the Centers for Disease Control and Prevention reported Thursday. The race left them infected with campylobacter coli, a common bacteria that causes a week of diarrhea, cramping, abdominal pain and fever, two to five days after exposure.

Usually, that bacteria gets into people who eat raw or undercooked poultry. But public health officials reported in the CDC's Morbidity and Mortality Weekly Report that they found a "statistically significant association" between accidentally swallowing muddy water during the race and the four confirmed and 18 suspected cases of infection. They think the water was laced with cattle or swine feces.

Obstacle racing, also known as adventure racing, has enjoyed rapid growth in recent years. Participants run various distances, climbing walls, crawling beneath barbed wire, dodging live electrical wires and leaping fire pits. Crawling through mud and submerging in water are staples of the events.

I did one of these races in 2010, and while I had to throw away my clothes afterwards, the biggest threat I faced was losing my shoes in ankle deep mud during a lake crossing. Fortunately I had duct-taped them to my ankles. But I can confirm that it's very difficult to do even a 5K without getting water or mud on your face, if not in your mouth.

Obstacle racing has built a big following since then, and the risk of injury and macho swagger are part of the marketing pitch many of the races make to men and women, mostly in their 20s and 30s, who are stuck in front of computer screens all week. Indeed, one 48-hour version calls itself the Spartan Death Race and warns participants that they may not survive.

Some have not. An Ellicott City, Md. man drowned during a Tough Mudder in West Virginia in April 2013, and two others died at a Warrior Dash in Kansas City in 2011. Others have been paralyzed and had heart attacks, though statistically such injuries are still rare.

Lt. Col. Chad Claar, the public health flight commander at Nellis Air Force Base who led the research, said he is aware of two more cases and suspects there were many more among participants in the Tough Mudder race, a lot of them military personnel. Typically, the 22 cases found would represent just the tip of the iceberg of an outbreak such as this, he said. Other people probably sought treatment from private doctors or didn't become sick enough to require care, he said.

The usual treatment is to make sure a patient is hydrated and not suffering complications, because by the time the symptoms appear, it isn't worth administering antibiotics, Claar said. The condition tends to run its course on its own.

Claar said obstacle racers should "know some basic public health prevention," such as good hand-washing after such an event. "If you’re going to be exposed to water in the environment, make sure you’re not drinking" from an unapproved source, and if you submerge in water, make sure you don't get water in your mouth or eyes, he said.

The researchers found two similar outbreaks associated with bicycle races in Norway in the 1990s and two more during mountain bike races in Wales in 2008 and British Columbia in 2010.

They suggested that participants be warned about ingesting water or mud just as they are notified of the possibility of injury when they sign up for the events.

Speaking from the heart, and the colon, I agree.