system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the role of cold temperatures in to make people more susceptible to getting sick eg. “catching a cold”?

When I was a child, my mother would tell us not to be in the cold for too long as we would “catch a cold” — but it seems inconsistent with germ theory..
gpt
Ooh, this one is interesting and multifaceted. It's not actually being out in the cold itself that does it, although there is some evidence that cold weather hampers our immune response.

But for one thing, most viruses replicate and survive more easily at cold temperatures. It's why the flu seems to disappear in the summer and why a fever can be helpful. A viral particle in cold weather stays infectious for longer.

Winter means fewer hours of sunlight, which means less chance to boost your vitamin D production, which can reduce your immune response.

And to top it off, cold air holds less moisture, and the heaters we use inside dry the air out further. This dries out your sinuses, which makes them less effective at trapping viral particles - and considering we all spend more time inside during the winter, that means extra time for you to sniff up a viral particle sneezed out by your family member or to pick up some snot your friend who came in with a wintery running nose smeared on some surface.