West Nile virus is a virus of the family Flaviviridae, found in both tropical and temperate regions.
It mainly infects birds, but is known to infect humans, horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits.
The main route of human infection is through the bite of an infected mosquito.
Image reconstructions and cryoelectron microscopy reveal 50-nm virions covered with a relatively smooth protein surface.
This structure is remarkably similar to the dengue fever virus.
In most people (80%), infection causes no symptoms.
In others, the virus causes mild flu-like symptoms known as West Nile fever.
The virus is able to pass the blood-brain barrier, and the most serious effects (in 0.7% of the infected) are encephalitis (inflammation of the brain) and meningitis (inflammation of the lining of the brain and spinal cord), both of which can be fatal.
Persons over 50 years of age are at higher risk of developing these more serious complications.
In rare cases patients may develop temporary blindness which can last from one to four weeks.