Most have no symptoms. Most people infected with the West Nile virus have no signs or symptoms. Mild infection signs and symptoms. About 20 percent of people develop a mild infection called West Nile fever. Common signs and symptoms of West Nile fever include:
Signs and symptoms of serious infection occur in less than 1 percent of infected people. Such infection may include inflammation of the brain (encephalitis) or of the brain and surrounding membranes (meningoencephalitis).
Serious infection may also include infection and inflammation of the membranes surrounding the brain and spinal cord (meningitis), inflammation of the spinal cord (West Nile poliomyelitis) and acute flaccid paralysis - a sudden weakness in your arms, legs or breathing muscles. Signs and symptoms of these diseases include:
Signs and symptoms of West Nile fever usually last a few days, but sign and symptoms of encephalitis or meningitis can linger for weeks, and certain neurological effects, such as muscle weakness, may be permanent.
When should you see a doctor?
Mild symptoms of West Nile fever usually resolve on their own. If you experience signs or symptoms of serious infection, such as severe headaches, a stiff neck or an altered mental state, seek medical attention right away. A serious West Nile virus infection generally requires hospitalization.
From Mayo Clinic