I read that being in a cold environment weakens the immune response in your nostrils when breathing in that cold air enough to cool off the inside of your nose, where then the pathogen fails to be destroyed as it would have normally had it not been cold.
I recently saw an article on HN (don't have a link, sorry) that said that cold did not, in fact, do anything to make us sick or help us catch stuff[1], EXCEPT that it made it more likely to spread something due to running nose or other contact with body fluids.
[1] it may have been specifically about cold and flu, I don't remember exactly. It was within the past month or two in case you feel like searching...
Air circulation is compromised in the winter because we close all the windows and doors, and generally we congregate closer together.
For a long time that was considered to be the only reason colds happen more in cold weather. This thing with influenza viability would change that (of course, what one study finds another refutes, so who really knows).
Here in Florida no circulation season is April - November. And yet flu season is still winter, the only part of the year where people even consider opening windows.
Influenza is caused by a virus, and viruses tend to be extremely resilient. They're not alive so temperature changes don't affect internal chemical reactions as it would for bacteria and humans.
Bacteria have temperatures at which they grow most effectively, which is why we try to keep food cold. Note that the food in your fridge still goes bad, and it would be pretty damn uncomfortable living at fridge temperature for humans. Bacteria are way more resilient than we are, though it varies by species.
> Influenza is caused by a virus, and viruses tend to be extremely resilient.
Viruses can be extremely resilient or extremely fragile, depending on a number of factors including whether or not they're enveloped, whether or not they're currently protected in an aerosolized droplet, etc.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4463890/