Every year, the flu almost goes extinct in temperate places like the United States. The key word is "almost." It stays afloat by constantly moving.
"It looks like it's hopping between different cities and different populations," says Sarah Cobey, a computational biologist at the University of Chicago.
The virus does an annual migration across the world, hitting the Southern Hemisphere during its winter, the Northern Hemisphere right about now, and hanging out in the tropics in between — especially in parts of Asia.