The West Nile virus quickly spread across the United States after the
first reported cases in Queens, New York on this day in 1999. The
virus was believed to have entered in an infected bird or mosquito,
although there was no clear evidence. The disease spread quickly
through infected birds. Mosquitoes spread the disease to mammals.

The first human cases usually followed within three months of the
first appearance of infected birds in the area except where cold
weather interrupted the mosquito vectors.