All of this is based on what I experience and doesn't necessarily describe the true nature of Japan/japanese people.
If you had seen at least one documentary about Japan, you will have noticed that many people there seem to wear those masks that usually the doctors would wear.
Well, it's not true that many wear them.
Almost everyone does.
If I only take those working in the town hall then I would say that surely 58% of all workers there had one. In the super markets about 40% have one. And every third driver of a car wears them.
And - since one of my tutors wears one, too - I went and asked what is behind that.
Japanese society is based on a few rules and one of those is: don't bother others.
So they will wear a mask once the don't feel well. To hide a running nose, to prevent themselfs from making others sick or to simply feel better once the throat hurts and you don't know, if it was from that karaoke session last night or from a incoming cold. But those masks are white and no one paints on them except for children and freaks (otaku). The latter are stared at (even more then at a Gaijin [foreigner] like me) and avoided. Even to the point where the shop clercs greet them pretty softly in comparison to the enthusiastic greeting the other customers get.
But yeah, those masks are everywhere. They are also amazingly cheap (89 Yen) and come in all kinds of variations, from simple paper ones over those you can wash, some with a "window" and the high quality ones even seem to smell of something.
So I guess that it is obvious that I will need to get those sooner or later, too.
At least, if I don't want to make everyone run away in pure shock and disgust when I start coughing and don't wear one.
Fun fact: Sneezing - on the other hand - isn't a sign of incoming sickness here. It is absolutly ok to sneeze, even without holding your hand/arm in front of your face. But don't dare to comment on someone sneezing like "Gesundheit!" or "Are you getting sick?" or even comment your own sneeze. Because sneezing is very embarrassing, since you can't stop yourself from doing it. Japanese people will totally ignore your sneeze and you should do it, too.