A report on the research is being published on Friday in the journal Science.

A basic belief in evolutionary biology holds that genes survive over thousands of generations only if they confer some advantage to the species. If a gene confers a disadvantage, like a lethal disorder, the theory is that it will eventually disappear from the gene pool because carriers of the gene would not have the opportunity to reproduce as frequently as those who did not carry the flawed gene. But the CF gene has not obeyed this rule. About 1 in every 20 Caucasians carries a CF gene. A single copy of the gene does not cause the disorder.

When two carriers become parents, however, their children have a 1-in-4 chance of inheriting two CF genes. This would cause cystic fibrosis, a disorder that until recent decades was nearly always lethal at a young age.

Cystic fibrosis kills because it prevents the normal flow of salts out of the gut and lungs. This leads to formation of a thick mucus that can obstruct the intestine and block breathing in the lungs.

Dr. Gabriel said the CF gene protected carriers against cholera because it blocked the same molecular pathway used by the disease toxin to cause diarrhea.

Cholera kills by causing a severe and unrelenting loss of fluid. Most of the disease victims die from dehydration. Some victims can lose up to a pint of fluid an hour from uncontrolled diarrhea.