Answer: This map shows the number of West Nile Virus cases in the United States during 2007, as reported to the Centers for Disease Control (CDC). West Nile Virus is transmitted through mosquito bites and can cause encephalitis (swelling on the brain) and death in humans, horses and avians (birds). It first appeared in the United States in 1999 and quickly spread throughout most of the lower 48 states by 2003. Epidemiologists at the CDC map the number, type (human cases vs. avian, animal, or avian infections) and location of West Nile Virus activity each year. This helps determine if the disease is spreading and how quickly. Epidemiologists then look for risk factors that correlate with a high disease prevalence in states like Montana, Texas and California. Some potential risk factors include a person’s age (your risk of infection increases with age), the amount of time a person spends outdoors (especially in places with many mosquitoes) and the location of major bird migration flyways.