Now and then you see something like this when looking at pictures from Japan:
There are supposedly many reasons for this, but one is, quoting a summary from Wikipedia:
Surgical masks are popularly worn by the general public in East Asian countries to reduce the chance of spreading airborne diseases; in Japan, it is common to wear a face mask whilst ill to avoid infecting others in public settings. In Japan and Taiwan, it is common to see these masks worn while ill, as a show of consideration for others and social responsibility.
My question is: Does it help? That is, are people in Japan less sick from airborne diseases compared to cultures where it is not common to wear a mask? Are there any data to back it up, or to disprove it? I'm specifically thinking of the common cold, but other diseases would be interesting.
I appreciate the difficulty in answering such a question - it seems as if you would have to find a near-identical country to compare with, but then again, there are probably a lot of organizations tracking just this sort of thing.