The heart problem blamed in the death of a Chicago marathoner is rarely dangerous and people who have it shouldn't stop exercising, heart experts said.

"People are not dying in the streets because they have mitral valve prolapse," said Dr. Phillip Watkins a cardiologist with a Birmingham, Ala., center that specializes in treating the problem.

The condition is a birth defect that affects 2 to 5 percent of the population, heart experts said. Many people don't know they have it until a doctor hears a distinctive "clicking" through a stethoscope.

The mitral valve controls blood flow on the left side of the heart, allowing it to flow from the upper heart chamber to the lower one. In severe prolapse cases, the valve leaks blood backward into the upper chamber, forcing the heart to work harder. Over time, damage can occur.

It can be corrected with minimally invasive surgery, but most people don't require it, said Dr. Tomislav Mihaljevic, a Cleveland Clinic heart surgeon.

"This is a common condition that often affects younger individuals," Mihaljevic said. "If it's mild, it requires no treatment. If it's severe, an operation can be done with low risk."

Cook County Chief Medical Examiner Dr. Nancy Jones said Tuesday that 35-year-old Chad Schieber, a Michigan police officer and father of three, had "significant mitral valve prolapse." An autopsy found the death was not related to heat stress, and test results for dehydration are pending, her office said Tuesday.

Experts were hesitant to comment directly on Schieber's case, but many said they hoped others with the condition wouldn't be frightened by the death.

Dehydration may have contributed to Schieber's death, Watkins speculated, stressing that he doesn't know the specifics of the case. The victim's family said Schieber encountered at least one water station that had no water.

Even running a marathon on a hot, humid day wouldn't be extraordinarily dangerous for most people with the condition, assuming they trained and stayed hydrated during the race, experts said.

Dr. Matthew Sorrentino, a cardiologist at the University of Chicago Medical Center, said athletes with any type of heart condition can be more susceptible to heat and dehydration.

"Mitral valve prolapse may be an innocent bystander in this case," Sorrentino said.

Muggy weather prompted officials to stop Sunday's race after about 3 1/2 hours. Runners reported shortages of water and sports drinks at aid stations. Volunteers couldn't always keep up with the demand, marathon officials said.

Schieber was diagnosed with the heart problem two years ago, his family said. He had a physical before he started training and his doctor had cleared him to run in the 26.2-mile race.

He was so physically fit that he helped train police officers to become bicycle patrolmen, said Krystn Madrine, his sister-in-law.

Schieber and his wife, Sarah who ran in the marathon with him, encountered a water station that was out of water, Madrine said.

"His body was under more extreme stress than anybody's should be under," she said.

Rarely, someone with the condition can experience a sudden rupture in the delicate fibrous tissue that connects the valve to the heart muscle, leading to severe leakage of blood into the lungs, said Dr. Robert Bonow, chief of cardiology at Northwestern University's Feinberg School of Medicine.

"That can be fatal, especially under extreme circumstances, such as exertion or hot weather," Bonow said