Since then Clark, who suffers from the sickle cell trait, hasn't played when the team travels to the high-altitude, midwestern state. He will miss his team's playoff game there Sunday as well.
Although rare, high altitudes can worsen complications of the trait because there is less oxygen, Lanzkron said. She said nobody knows why altitude cause complications in some people and not others.
In Ryan's case, it lead to severe pain in his left side after a game in Denver on Oct. 21, 2007. After some tests his spleen and gallbladder were removed and his weight dropped from 205 pounds to about 170. He missed thr rest of the season.
He also did not play in Denver during a regular season game in 2009 and a preseason game in 2010.
Most people with sickle celltrait lead a normal life because only 40 percent of their cells are affected by the disease. Those with sickle cell disease have shorter lifespans because the disease attacks more cells.
There is some evidence that those with the trait could suffer sudden death if they work out too hard, Lanzkron said. But she wouldn't go as far as to tell someone with the trait not to play sports.
She said many people probably don't know they have the sicke cell trait and it is hard to tell how many NFL players also have the disease.
Sickle cell disease affects 8 to 10 percent of the African American population.