He sent samples to labs in Wisconsin and Ohio which both determined that they had not seen this disease before. The virus is named for where it was first identified in 1937 when the first person died of it in Uganda. This particular strain of West Nile was identified in Israel a year before coming to the US, in 1998. Crows are very sensitive to West Nile. There are only theories.
Mosquitoes carry the virus and transmit it from birds to humans and back again.
One theory is that an infected mosquito got on a plane to New York. Possibly a mosquito hitched a ride on a ship or tanker. Though the birds are quarantined for seven weeks, it is possible that it came in a bird that only had a very mild illness, and it went unnoticed. Perhaps the virus arrived in a zoo or pet mammal. 1999 was probably not year one for West Nile virus in New York.