Why do Japanese people wear medical masks in public?

Over the last 30 years, I've been to Japan six times. The first time I went I saw a few people wearing medical masks that cover their face and nose. I was told they had a cold and didn't want to spread germs. On my subsequent visits to Japan, more and more people were wearing masks. The last time I went I saw masks everywhere I went. Every time I walked on the street, went into a shop, took a subway, or walked in the hills a mask-wearer was in view. In this video, people are asked why they wear masks. Reasons include: pollen allergies (one guy with pollen allergies was wearing a mask over his mouth but not his nose because he didn't want to fog his glasses. "I know it doesn't work, but I wear it anyway."), polyp in the throat, slight cold, to warm the face in winter, runny nose, "without make-up, my face looks ugly" (said by a man).