Someone suggested to me once that the reason cold weather tends to be cold & flu season is simply becayse when it’s cold out, people spend more time indoors with each other, cozily sharing their germs. When the weather’s warmer they spend more time walking around outside, dispersing the germs on the winds. Not a very detailed answer, but plausible as a contributing factor at least.
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This doesn’t explain why colds are often more common during the winter months in the tropics, too, where people are still outside a lot and the weather doesn’t vary much.
Throughout the entire tropics, or just the northern half? How about in the southern hemisphere? Is their cold and flu season during our winter or theirs?