system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why we're more likely to get ill during colder seasons

Why is it that whenever winter-time comes, people suddenly start getting ill? How does cold affect our immune system?
gpt
There are many compounding factors. The strength of your immune system isn't really one of them, actually.

The biggest is that we tend to clump up inside in cold weather. So there are more people in less space sharing germs.

Sunlight and heat are both good at killing viruses as well, so viruses live longer on surfaces indoors.