West Nile Virus (WNV) was first discovered as a cause of an infectious disease in Uganda, Africa in 1937. It then spread to the Middle East, Eastern Europe, West Asia and, more recently, the United States. It is a very uncommon condition in the UK, with only 2 cases recorded since 2002, both of which being associated with travel to Canada.
WNV belongs to a group of disease-causing viruses known as flaviviruses. These lipid (fat)-enveloped viruses can be spread by insects, usually mosquitoes, to animals, including humans. Most human infections are mild, although in some cases serious illness such as encephalitis can result from infection. It is not contagious person-to-person.
The most common route of transmission is through a mosquito bite. When a mosquito bites an infected bird, such as a crow (crows are highly susceptible to infection), it can then transmit the virus to another animal it bites. Transmission of the virus commonly peaks during early spring when adult mosquitoes emerge and continues until autumn. It is estimated that 1 in 200 mosquitoes in the US harbours the virus.
There also has been some recent concern about transmission of WNV through donated blood or organs, as seems to have occurred in the case of four organ transplant recipients.