Infectious disease. The enigma of West Nile. .

Fifteen months after the 1999 outbreak of West Nile virus in New York City, which sickened 62 mostly elderly people and killed seven, scientists are still hard pressed to predict how abundant the virus will eventually become or how serious a public health threat it will pose. This summer, the human toll has been relatively mild, with just 18 cases and one death. But the virus has been found in more than 60 bird species and about a dozen mammals; in a little more than a year, it has spread to 11 states along the East Coast and the District of Columbia. And with no natural barriers to stop it, scientists can safely say that it will keep spreading.