[galaxyLogic]

It's a bit counter-intuitive that it is precisely in the presence of people who don't wear a mask that YOU should be wearing one.

Our intuitive understanding, inclination is "do as others do". "Oh, nobody's wearing a mask, must mean there is no virus around here". No. Not. If people are not wearing masks it is more likely that there's a lot of virus going on.

People who don't wear a mask in your presence are likely to be careless about protecting them against the virus in other ways, and therefore are more likely to have it already.