According to the Associated Press' Joe McKay, Bengals defensive tackle Geno Atkins was born with sickle cell trait. His father, Gene Atkins Sr., a ten-year safety with the New Orleans Saints and Miami Dolphins, was affected by the same thing -- and that didn't stop him from recording 25 career interceptions, including 198 return yards in 1991 that led the NFL.
After extensive research into learning about sickle cell trait (aka, Wiki search), "Sickle cell trait (or sicklemia) describes a condition in which a person has one abnormal allele of the hemoglobin beta gene (is heterozygous), but does not display the severe symptoms of sickle cell disease that occur in a person who has two copies of that allele (is homozygous)."
One of several complications that may arise include "reports of pulmonary venous thromboembolism" during prolonged air flights."
The affliction made headlines last year when it was learned that Steelers defensive back Ryan Clark had to sit out in a game at Denver. Santonio Holmes is known to have the same thing.