The cold weather makes your immune system work harder, so all the normal crap you get exposed to has a higher chance to slip through the cracks and start growing.

The factor about people “huddling together” is a really bad way to say that generally people go outside less, and more time inside increases the chances of being around other people, and even if those people are not actively sick they can still be carrying enough virus to spread it. Think of it like getting sun burned, you can get exposed and be fine, some people are really good at resisting exposure, others are more sensitive, but if you do nothing to reduce exposure and nothing to protect yourself from that exposure, ya gonna get burned.