They are not supposed to protect against a virus so much as to keep the virus with the infected wearer. (Although they would be bound to catch some from others.)

The expressed purpose of a mask is to trap water particles/droplets (saliva, mucus, snot) coming out of the wearer’s mouth or nose, with the viruses in the droplets stopped at the same time. It isn’t thought to be perfect but less likely for one who is infected from infecting others. People may be infected and never know it and still able to infect and possibly kill others.

And if possible, just keep some distance between you and others makes infecting others less likely.

David for better neighbors.