There's new research that infrared radiation (mostly experienced when it's sunny and you're outside) helps mitochondrial function and boosts the immune system.

So there's at least 4 mechanisms:

- warmer = less likely to catch a cold

- more time outside = more UV which kills pathogens

- more time outside = less time spent exchanging pathogens with other people

- more time outside = more infrared radiation which boosts the immune system