West Nile virus is an arthropod-borne virus (arbovirus) that was first identified in the West Nile district of Uganda in 1937, and has since been spreading worldwide. Currently, the virus has been found in Africa, Asia, Europe, the Middle East, and North America.

West Nile virus is transmitted by bloodfeeding insects such as mosquitoes, and perhaps by ticks. Avian species, especially wild birds, act as reservoir hosts, but the virus also can infect many other species as the end host. This includes humans, horses, dogs, cats, bats, chipmunks, skunks, squirrels, domestic rabbits, and domestic and exotic birds.

Mosquitoes contract the virus by feeding on infected birds. The level of viremia is too low in species other than birds to infect mosquitoes. Infected mosquitoes transmit the virus to humans and animals via bites, causing West Nile viral encephalitis once the virus has multiplied and crosses the blood-brain barrier.

Many avian species, especially crows, harbor the virus with very high viral loads but without clinical signs of illness. Viremia usually occurs 1-4 days post-infection, but once recovered, immunity is life-long. An important early epidemiologic sign of the presence of West Nile virus in a region may be loss of birds (reduced numbers, ill or dying birds).

In horses, viremia occurs 5-7 days post-infection; and they are highly susceptible to clinical disease. Recovered cases are immune for life. Rabbits, rats, and guinea pigs are resistant to the virus, whereas the status of mice and hamsters is unclear. Dogs seroconvert upon exposure but are usually asymptomatic. Cats also seroconvert but rarely show clinical signs of illness. Humans and other primates can become infected and can develop encephalitis.

There is no documented evidence of animal-to-animal or animal-to-person transmission of West Nile virus. However, human-to-human transmission has been reported through organ transplantation or blood transfusion, and so this could be a means of transmission between animals. People, like veterinarians, who directly handle sick animals may be at risk and should take precautions to avoid contact with their saliva.

In areas where mosquitoes are known to carry the virus, less than 1% are actually infected. Even when people are bitten and infected by these mosquitoes, less than 1% become severely ill. Thus, the risk of becoming ill from a single mosquito bite is very low.