Flu season is in full swing across the U.S. this week, and it's right on time. Actually, it would have been on time if it showed up in November, too, or even in March--unlike the actual seasons, the timing of flu season is a little hard to predict. Except for one thing: it always happens in winter. In fact, everywhere on Earth where people have a winter season, they also have a flu season:

Part of what makes the timing of each flu season unpredictable is that scientists still don't understand exactly why we have one at all. There have, of course, been lots of theories:

One theory is that flu peaks in winter because people spend more time indoors, with the windows closed, breathing each other’s air: