The westward expansion caught officials from the Centers for Disease Control (CDC) by surprise; they expected the trail of human infections to follow the migratory patterns of birds. Could birds flying south for the winter have strayed off course? Or is the disease which is transmitted to humans by mosquitoes that have bitten infected birds being spread by a new breed of mosquito?
Fortunately, the disease is relatively hard to catch. Fewer than 1% of humans who are infected actually get sick, and it does not spread from one person to another. But in a small percentage of cases, particularly those involving the elderly, it can be fatal. The virus causes flulike symptoms within three to 15 days and can lead to a dangerous inflammation of the brain.
The CDC predicts that West Nile will continue its flight westward and eventually spread to every state, except maybe Hawaii and Alaska. Since there is no vaccine or treatment, the best protection is to stay indoors at dawn and dusk, when mosquitoes are most active, and douse exposed skin with insect repellent.