West Nile virus is transmitted by mosquitoes, who get it by biting infected birds, according to the Centers for Disease Control and Prevention.
About four out of five people who become infected by a mosquito bite have no symptoms.
However, about 20% of people who become infected with West Nile virus develop West Nile fever, within two to 15 days afterward.
Symptoms include fever, headache, tiredness, body aches, occasionally with a skin rash (on the trunk of the body) and swollen lymph glands. While the illness can be as short as a few days, even healthy people have reported being sick for several weeks.
There is no treatment for West Nile virus fever, the CDC says.