Part of the human respiratory function is to expel germs and crap through our oral and nasal passages. The mask blocks this expulsion, trapping the germs so the masked person can breathe them in again.
Not just sneezing and coughing, but normal breathing expels things.
So now what?
So you're worried about the germs already in your body going back in to your body?
Te occidere possunt sed te edere non possunt nefas est