IN LATE AUGUST of 1999, several physicians in the New York City area notified the New York City Department of Health about eight patients with an unusual type of encephalitis (inflammation of the brain). Doctors known as epidemiologists, who specialize in tracking down the causes of and controlling mysterious illnesses, began looking at what these patients had in common.

They found that all eight were previously fairly healthy.

The patients ranged in age from fifty-eight to eighty-seven. All had fevers followed by changes in mental function. Seven of the people had severe muscle weakness, which is unusual with encephalitis.

Three had symptoms of Guillain-Barré syndrome, a disease characterized by sudden weakness or paralysis of the arms, legs, face, and breathing muscles. Four were paralyzed to the extent that...