West Nile virus is caused by a virus that is commonly spread by infected mosquitoes. Mosquitoes get infected after biting infected birds. While West Nile virus is usually found in other parts of the world such as Africa, the Middle East, India and parts of Asia, the virus is present in the United States as well. There are about 1,000 cases of West Nile virus reported in the United States each year. However, these numbers can vary.