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 include:

• Fever
• Headache
• Body aches
• Vomiting
• Diarrhea
• Fatigue
• Skin rash

Serious infection signs and symptoms

In less than 1 percent of infected people, the virus causes a serious neurological infection, including inflammation of the brain (encephalitis) and of the membranes surrounding the brain and spinal cord (meningitis).

Signs and symptoms of neurological infections include:

• High fever
• Severe headache
• Stiff neck
• Disorientation or confusion
• Stupor or coma
• Tremors or muscle jerking
• Seizures
• Partial paralysis or muscle weakness

Signs and symptoms of West Nile fever usually last a few days, but signs and symptoms of encephalitis or meningitis can linger for weeks or months. Certain neurological effects, such as muscle weakness, can be permanent.

Typically, West Nile virus spreads to humans and animals via infected mosquitoes. Mosquitoes become infected when they feed on infected birds. You can't get infected from casual contact with an infected person or animal.

Most West Nile virus infections occur during warm weather, when mosquitos are active. The incubation period — the period between when you're bitten by an infected mosquito and the appearance of signs and symptoms of the illness — ranges from two to 14 days.

West Nile virus has occurred in Africa, Asia, Europe and the Middle East. It appeared in the United States in the summer of 1999, and since then has been reported in every state except Hawaii and Alaska, as well as in Canada.

Other possible routes of transmission

In a few cases, West Nile virus might have spread through other routes, including organ transplantation and blood transfusion. However, blood donors are screened for the virus, substantially reducing the risk of infection from blood transfusions.

There also have been reports of possible transmission of the virus from mother to child during pregnancy or breast-feeding or exposure to the virus in a lab, but these are rare and not conclusively confirmed.

Most cases of West Nile virus in the United States occur June through September. Cases have been reported in all 48 lower states.

Risk of serious infection

Even if you're infected, your risk of developing a serious West Nile virus-related illness is extremely small — less than 1 percent of people who are infected become severely ill. And most people who do become sick recover fully. You're more likely to develop a severe or fatal infection based on:

• Age. Being older puts you at higher risk.
• Certain medical conditions. Certain diseases, such as cancer, diabetes, hypertension and kidney disease, increase your risk. So does receiving an organ transplant.