The West Nile virus quickly spread across North America after its introduction in 1999. It is theorized that the infection was introduced accidentally by an air traveler who was infected before he arrived in New York.
The disease spread quickly through infected bird, as the mosquitoes spread the disease to mammals including humans. As of 2007, 47 of 48 mainland states in the US have cases of West Nile virus infections, 12 of which had greater than 10% mortality rates in certain years.
West Nile virus was at its deadliest in the United States in 2003, with close to 10,000 cases of infections, 264 of which were fatal. However, cases of infection seem to have dwindled recently, largely due to increased awareness and improved immunity (although it can be said that there are a lot of mild cases of West Nile infection that go undiagnosed).
Aside from bites from infected mosquitoes, some mild cases of infection were discovered during blood donation screenings. There were 30 cases of West Nile virus infection from blood transfusion in the United States, majority of which were acquired from 2002 before blood screening was instituted.
In September 2002, American researchers reported the first polio-like paralysis stemming from West Nile virus. Infectious disease specialists in Ontario, Canada, began seeing West Nile patients hooked up to ventilators as they are unable to move or breathe. Such cases of West Nile virus infection affect the central nervous system and infects neurons and brain stem regions within the individual.
The October 2002 issue of New England Journal of Medicine shows results of research by the US Centers for Disease Control and Prevention. They say that for every five people infected with West Nile virus, one has a mild illness that usually lasts for three to six days.
Meanwhile, meningitis or encephalitis would occur in one in 50 people infected with the virus, more commonly in those over age 50. The researchers suggest severe muscle weakness as a common symptom that may offer doctors a diagnostic clue.