In response to outbreaks of West Nile virus throughout the eastern U.S., the Centers for Disease Control and Prevention has pointed out that the best way to avoid infection is to avoid mosquito bites. West Nile virus is spread by the bite of an infected mosquito, and in addition to humans can infect horses, many species of birds, and some other animals. Fortunately, most people who become infected with West Nile virus will have either no symptoms or mild ones. But on rare occasions, infection can result in West Nile encephalitis, a severe and sometimes fatal inflammation of the brain. (The risk of severe infection is higher for persons 50 years of age and older.) Officials in Washington D.C. have been getting the word out about protective measures people can take after West Nile virus infected a 55-year-old resident, who is now hospitalized with encephalitis. As of Thursday, state health departments around the U.S. have released information on 113 cases of human illness related to West Nile virus this year, including 5 deaths.