WNV can infect humans, birds, mosquitoes, horses and some other mammals, but is most common in birds. It is found in Africa, West Asia, Australasia, the Middle East and more recently, in North America, where in 2003 it was responsible for over 250 deaths.

Spread by the bite of an infected mosquito, in humans it generally produces only a mild illness, but like many other arboviruses, can produce serious infection of the brain, or encephalitis.

The incubation period is between 3 and 15 days,however 80% of those infected will not experience any symptoms. 20% will develop flu-like symptoms, including fever, headache and general aches and pains, which usually last between 3 and 6 days. Less than 1% of people infected with WNV will develop severe infection leading to encephalitis, which can be fatal. Severe infection is more likely in people over 50 and those with a weak immune system.

Seek professional medical advice prior to visiting a high risk area. The best protection against this virus is bite prevention, see our section on prevention.

Travellers should alert their GP of any fever illness occurring after leaving an endemic area.