The study by scientists with the National Institute of Allergy and Infectious Disease was published in the online edition of the Journal of Experimental Medicine.
The mutation is the first genetic factor found that increases the risk of becoming ill from infection with the West Nile virus. The virus does not necessarily make infected individuals sick but can cause potentially fatal symptoms.
A protein, CCR5, helps the HIV virus spread from cell to cell. In the United States, about 1 percent of whites have a mutation that causes them to produce no CCR5, making them effectively immune to HIV.
The researchers used genetically engineered mice lacking in CCR5 and found that they became fatally ill when infected with West Nile.
Dr. Philip Murphy, the lead researcher, said that the findings have implications for AIDS treatment. For example, patients taking CCR5 should do their best to avoid mosquito bites, since those are the primary source of West Nile infection.