The virus originated in Africa but has been spread northward by migratory birds and mosquitoes. There have been several outbreaks in southern and central Europe in recent years. It is found in much of the U.S.
Jonas Schmidt-Chanasit of Germany's Bernhard Nocht Institute tropical medicine center said that "the unusually warm summers of the last two years, triggered by climate change, apparently have contributed to WNV establishing itself north of the Alps."
The virus causes dangerous meningitis or encephalitis in less than 1% of those infected.