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.
西尼罗河病毒源自非洲，但通过候鸟和蚊子已向北传播。近年来，欧洲南部和中部已爆发多次。美国大部分地区也有病例确诊。