“For the coming on of winter resolves the diseases of summer, and the coming on of summer removes those of winter,” wrote the father of medicine, the Greek physician and philosopher Hippocrates, in 400 B.C.

We still haven’t totally figured it out, but researchers now understand that as the seasons cycle, germs, their hosts, and the environments they inhabit change together in complicated ways that make people more or less prone to catching a disease.

“If you asked five flu epidemiologists what they thought the relative contributions are, you'd have a decent chance of getting five different answers,” Dylan Morris, a postdoctoral researcher at UCLA studying the ecology and evolution of viruses, says with a laugh.

That said, scientists have discovered how