West Nile virus belongs to the Flavivirida family and is in the same phylogenetic group of flaviviruses as encephalitis viruses. A complex of viruses that are found both in tropical regions, as well as in temperate ones. The virus primarily affects birds but can infect people, horses, dogs, bats, monkeys, squirrels and house mice. In humans, the virus is transmitted through the bite of a mosquito. In 90% of cases, West Nile virus infections in humans are asymptomatic.