In the early days of the pandemic they were citing epidemiologists who said the something or other number was about 2.4 ie. each person had infected on average 2.4 other people and that's why the virus spread so fast. If this number is less than 1.0 the virus dies out.

So let's say, just for discussion, if I wear a mask it only reduces this number by 30%, so now I'm only infecting 1.7 others. Then if you also wear a mask it protects you, maybe not so much, but let's say it reduces the number by 25% so now it's down to 1.3 or so. Then if we maintain 2 meters between us and that attenuates the droplets by 30% it brings the number down to 0.88 which means the disease should eventually die out. I don't know the real fractions but this might be representative.

A mask is not 100% effective, but worth doing. The whole thing is probablistic, whether or not you get sick; except that the human body can reject a certain number of the virums or whatever the singular form of the word is. If not so, anyone who encountered one virum would always die. In reality most people only have minor symptoms or if they get sick most do recover. We wear masks basically for the minority that would die.