West Nile Virus (WNv) is spread by infected mosquitoes. In most cases, mosquitoes become infected when they feed on infected birds. Infected mosquitoes can then spread West Nile Virus to humans and other animals when they bite.
According to the Center for Disease Control (CDC), 4 out of 5 people infected with WNv will not show any symptoms at all. Some people (up to 20 percent) may experience mild symptoms, including fever, headache, and body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms can last for as short as a few days, though even healthy people have become sick for several weeks.
About 1 in 150 people infected with WNv will develop severe illness. The symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremor convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent.
People typically develop symptoms between 3 and 14 days after they are bitten by the infected mosquito.
Center for Disease Control Fact Sheet on West Nile Virus