During the past several years, we have experienced some global epidemics, such as sporadic spread of bird flu and the recent swine flu. In most countries, it was very noticeable how a lot of people start wearing face mask in order to prevent them from contracting the virus. Similarly in Japan, face mask was quite common, however, the culture was very different.
In Japan, those who wears the face mask are the ones that has contracted the flu. This was very different from other countries, including other Asian countries that I know of (like Indonesia and Hongkong).
When you think of it though, it was actually quite a common sense. Influenza virus spreads via respiratory droplets, which happened when you cough, sneezed or talk to other people. These droplets could enter through eyes, nose, or mouth over short distance (http://www.pamf.org/flu/spread.html). So, by wearing mask to prevent ourself from contracting the disease will probably not as efficient as to the other way around.
When I travelled to Japan, it was very noticeable that those with flu will always wears mask. Of course, few times I still see those who contracted flu didn't wear mask… and guess who they were? The foreigners of course =D…