Well, I reckon that you wear a mask to protect other people, not yourself - last I heard, a mask gives you 55% protection, but gives 65% protection or more to the people around you if you're infected but don't know it yet, by stopping the virus-laden aerosols from getting into the air.

So you have a relatively simple way of telling idiots (people who only care about themselves) from decent people. If they're wearing a mask, they're decent.

If they're not...

Yes, I know about people who have "medical conditions" that prevent them from wearing masks - but surely, if you have a respiratory disorder that serious, you should be shielding at home anyway.

By the way, I really hate wearing a mask. But if you care even a little bit about other people, you'll do it. It's cheap, it's simple and it works OK - what more can you ask?

I avoid people who won't wear masks. And actually, I reconsider my friendships with people who won't wear them because I believe they've revealed themselves as fundamentally selfish.

As for people who won't self-isolate after they've tested positive - selfish isn't really the word. I believe they're criminally irresponsible. Unfortunately we can't make it illegal for people to knowingly wander around emitting great gouts of viral particles, because then the criminally irresponsible won't get tested. On that one, I'm as stumped as you are.