John H. Rappole and colleagues wrote this perspective article (for Emerging Infectious Diseases) on the potential implications of the spread of West Nile Virus by migrating birds. Although ornithophilic mosquitoes are the principal vectors of West Nile virus (in the Old World), several species of birds (mainly migrants) appear to be important introductory or amplifying hosts. Patterns of virus outbreaks in southern Europe are examined, and the authors suggest that "viremic migratory birds may also contribute to movement of the virus."