West Nile Virus is a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to St Louis encephalitis virus which is also found in the United States. The virus can infect humans, birds, mosquitoes, horses and some other mammals. The most severe type of disease due to a person being infected with West Nile virus is sometimes called 'neuroinvasive disease' because it affects a persons nervous system. Specific types of neuroinvasive disease include: West Nile encephalitis, West Nile meningitis or West Nile meningoencephalitis. Encephalitis refers to an inflammation of the brain, meningitis is an inflammation of the membrane around the brain and the spinal cord, and meningoencephalitis refers to inflammation of the brain and the membrane surrounding it. West Nile Fever is another type of illness that can occur in people who become infected with the virus. It is characterised by fever, headache, tiredness, aches and sometimes rash. This book presents the background of these virus and research results.