~~~ you do not spread your germs to others that you may come in contact with... and it's the same reason they wear masks in an operating room when standing over an open body during heart surgery, or whatever. The surgeons and staff don't wear the mask to prevent from "catching" heart disease or, they wear it so their germs do not fall into an open body... Just as the millions of people in Japan and all of Asia wear masks when they have the sniffles -- a courtesy extended to others.