Severe symptoms of West Nile virus include fever, confusion, convulsion, muscle weakness and vision loss, according to Healthline. Infected people may also experience numbness, paralysis and coma. Mild symptoms include headache, nausea, vomiting, swollen lymph glands and a rash on the chest, stomach or back.
Severe symptoms occur in one out of 150 infected people, according to the Centers for Disease Control and Prevention statistics cited on Healthline. Mild forms of West Nile virus may be confused with the flu. The severe form, which can last for several weeks, can cause permanent brain damage in rare cases. Approximately 80 percent of people that an infected mosquito bites experience no symptoms. Infected individuals typically exhibit symptoms within three to 14 days of being bitten.
Mosquitoes spread the virus by biting birds infected with the virus and then biting people, explains WebMD. West Nile can spread through an organ transplant or a blood transfusion although this is rare in the United States because all donated blood is screened for the virus. Some evidence suggests that West Nile can spread from a mother to her baby during pregnancy, at birth or through breast milk, but the risk of spreading the virus to babies seems to be very low.