This is the first reported outbreak of this strain in North America. The strain is very close genetically to St. Louis encephalitis, which is why the first cases studied were thought to be St. Louis encephalitis, said Rima Khabbaz of the National Center for Infectitious Diseases, part of the Centers for Disease Control and Prevention. It rarely causes disease in infected people, perhaps 1 percent of the time. And among the ill, it rarely proves fatal, perhaps 4.3 percent of the time, according to reports. That is a mystery, although migrating birds may be the culprits.
People get the virus from mosquito bites.
You can't get the virus from other people or other animals. The virus infects the central nervous system. Most infected people will not even feel sick. The majority of those who do will have only a mild fever, chills, a headache and aches and pains. But in severe cases, the brain or the meninges, the membranes that cover the brain, become inflamed.