The genes that showed fingerprints of natural selection were identified by two other authors of the study, Elinor K. Karlsson and Dr. Pardis C. Sabeti, computational biologists at Harvard, in the genomes of Bengalis, the majority population in Bangladesh.

The selected genes offer a road map to how the body defends itself against cholera. Toxin from the bacterium binds to cells in the victim’s intestine, prizing open the channels through which chloride ions leave the cell and forcing the cells to excrete volumes of water and electrolytes — up to two liters an hour. The diarrhea benefits the organism, which can reach drinking water and spread further, but can be lethal for the host if unchecked.

One set of selected genes in the Bengalis affects control of the innate immune system, the body’s first line of defense against microbes. Another set of genes affects the channels that control the flow of potassium ions in and out of cells. It is not yet known exactly how the variations in any of the selected genes affect the biology of resisting cholera. But variations in the potassium channel genes, for instance, could help reverse the outward flow of water caused by the cholera toxin’s attack on the chloride channels.

These findings fit the expected biology of cholera but upset other predictions. Cystic fibrosis is caused by a mutation in a gene that controls the movement of chloride in and out of cells. Since the cholera toxin requires this gene to be in working order, the mutation might be protective against it. But the researchers found no evidence of selection on the cystic fibrosis gene in the Bengali population.

People with blood group O are particularly susceptible to cholera, and indeed few Bengalis have blood group O. In scanning blood group genes, the researchers were not able to pick up any negative selection against the O group genes. But they unexpectedly found evidence of natural selection having favored a minor blood type known as the Kell blood group. The genes of this group have no known connection with cholera but seem to offer a defense against arsenic. The poison contaminates the groundwater of Bangladesh, having been released by the tube wells dug to secure clean water to protect against cholera. The presence of protective genes in the Bengali population, if verified, would suggest arsenic is not just a modern problem but has been unleashed by monsoons or other natural phenomena for thousands of years.