Jonah Triebwasser interviews Cary Institute disease ecologist Shannon LaDeau on the recently recognized dangers from the Zika virus and on the mosquitoes that carry it from human to human. Zika has been known in the Old World since the 1940s but did not reach the Western Hemisphere until 2014, when travelers brought it to Brazil. It has since spread widely through South America, the Caribbean, and some neighborhoods in Florida.