Just because someone isn't wearing a mask doesn't mean they don't want to. They might have a disability or medical condition you can't see that makes wearing a mask difficult or distressing.
How one of the most effective face masks could be putting others at risk.
A simple experiment suggests that we urgently need more research in this area.
It's nearly impossible to avoid close contact when protesting, and easy to forget the risks. An infectious disease expert answers key questions about how to avoid spreading the coronavirus to family.
They may not stop much viral spread, but they still do something important.