Well, usually only one person is sick at a time.
This would explain it better than I could.
I live in the US, and when I get a cold I'll wear a mask when I go out- 1 because I don't want to get anyone else sick and 2 because I don't want to get sicker. The strength of your immune system is lower when you're sick, so you're more likely to catch something else. No one wants two colds back to back!