Taken in isolation, the news reports that polio appears to have returned to Syria for the first time since the late 1990s, that dengue and yellow fever is showing up across the southern United States and that Texas has had its worst year ever for West Nile virus all seem like separate disease outbreaks. And they are. But taken together, they should also serve as a reminder that disease, especially infectious disease, doesn’t spread independent of human behavior – and bad behavior on the other side of the planet can kill here.