In either case, with mosquitoes as the vector, the virus soon spread. It's almost like a cockroach. But after matching genetic specimens of the human viral deaths to viral deaths in birds stricken at the Bronx Zoo, the CDC reclassified the virus. Many people might be infected and have no symptoms, and others might have a fever that goes away by itself. West Nile encephalitis often is accompanied by a skin rash and swollen lymph glands.
Severe infection can cause high fever, neck stiffness, disorientation, coma, tremors and paralysis.
The disease is estimated to kill 3 percent to 15 percent of its victims and is most dangerous to the elderly, the young and those with weakened immune systems. No treatment is available. The virus is not transmitted from person to person or directly from birds to people. Although birds of many kinds are believed to be carriers of the virus, its presence in crows has proved a particularly telling sentinel for the outbreak. Late last month, some dead crows were spotted near the Bronx Zoo.