West Nile Fever

West Nile fever is an infection caused by the West Nile virus, which spreads mainly through mosquito bites. About 80 percent of infections cause few or no symptoms, while roughly 20 percent produce fever, headache, vomiting or a rash, and in less than 1 percent of cases the virus reaches the nervous system and causes encephalitis or meningitis, with a death rate near 10 percent among those cases. The virus is usually carried by birds and picked up by mosquitoes that then bite people, and it can also spread rarely through blood transfusion, organ transplant, or from mother to child. It was discovered in Uganda in 1937 and reached North America in 1999, and there is still no vaccine for humans, though one exists for horses.