Washington ? Only about one in five people infected with the West Nile virus develop a severe, life-threatening illness. A study in mice suggests a gene variation may be the reason some become very sick from the mosquito-borne virus, while others recover easily.

Experts said the research is an important step toward finding a drug to treat West Nile, a virus that has caused 11 deaths in the United States this year. The Centers for Disease Control and Prevention reported Saturday there have been at least 251 human cases of the disease this year.

West Nile is carried by mosquitoes whose bite can spread the virus to birds, horses and humans. So far this year, the virus has been found in dead birds, in horses, in humans or in mosquito pools in at least 39 states, New York City and the District of Columbia. Human cases have occurred in 11 states and the District of Columbia.

For most people, West Nile causes only flu-like symptoms. But for some patients, particularly the young and the elderly, West Nile can be a killer, causing a swelling of the brain that can be lethal.

Just why some people are more susceptible to serious illness from the virus has intrigued researchers and prompted scientists at the Pasteur Institute in France to search for a gene variation that might explain the difference.

The French tested a series of laboratory mouse and found a genetic type that was most likely to die after being exposed to the West Nile virus.

The exact gene variation has not been found yet in humans, but researchers at the National Institutes of Health said the finding by the Pasteur scientists is important because it moves research closer to finding a drug for West Nile virus.

“The most promising thing about this study is that it could help lead to a drug” that would restore the missing protein and give patients protection against replication of the virus, said Catherine Laughlin of the National Institute of Allergy and Infectious Diseases at the NIH.