... get somebody's infected sputum in your mouth, nose, or eyes.
If absolutely everybody wore a surgical mask the chance somebody else's bodily fluids got inside of you would be greatly reduced to such a degree that those under 50 could get along with life (as long as they also regularly washed their hands)!!!
edit: How? By keeping that infected sputum from reaching the air or surfaces like a hankerchief put to a cough does.