West Nile virus is a flavivirus that is commonly found in Africa, Europe, the Middle East, North America, and West Asia and birds are the reservoir hosts (the virus is maintained in a mosquito-bird-mosquito transmission cycle). Transmission is mainly through mosquitos although people can also become infected through organ transplants, blood transfusions, and breast milk.

Infection is asymptomatic in 80% of cases, however, 20% of those infected will develop West Nile fever which can lead to severe neurological disease and death. Diagnosis is by RT-PCR or by detection of IgG and IgM antibodies to the virus in serum or cerebral spinal fluid.