West Nile virus is a mosquito-borne flavivirus, and maintained in mosquito-bird cycle in nature. Humans are a dead-end host in West Nile virus infection. Symptoms are developed in approximately 20% of West Nile virus-infected humans. Encephalitis and meningitis are developed in about 1 out of 150 infected humans, especially in elderly populations. It has recently been reported that West Nile virus causes polio-like, acute flaccid paralysis and polyneuritis. These symptoms occur in patients without central nervous system symptoms as well as in those with encephalitis. Thus, West Nile virus causes various types of illness in humans. West Nile virus has not entered Japan yet. However, West Nile virus should be considered to be one of the causative agents for patient with a febrile illness, meningitis, encephalitis or acute flaccid paralysis who came back from West Nile endemic or epidemic areas of the world.