How does WNV cause severe illness in humans?
Once a person is infected, WNV may multiply in the person’s blood system and cross the blood-brain barrier to reach the brain. Once there, the virus can interfere with normal functioning of the central nervous system and causes inflammation of brain tissue (encephalitis), with sometimes-fatal results. The risk of serious illness or death is greatest among the elderly or among individuals with weakened immune systems. It is important to realize that less than 1% of persons infected with WNV will develop severe illness. How long does it take for symptoms of WNV infection to appear? The incubation period for WNV infection in humans is usually 3 to 14 days. How is WNV transmitted? WNV lives in infected birds. Mosquitoes become infected when they feed on those birds. The infected mosquito can then transmit the virus to humans and other animals while biting to take blood. Once in the bloodstream, the virus may multiply and cause illness. There is some evidence that WNV infection can be trans
