Polite East Asian people with a cold may wear a mask with the aim of preventing its spread; this is especially common in Japan.
In China, the idea may be to cut down infection or it may be a reaction to high levels of visible pollution; in some of China’s industrial areas, masks are worn simply in the hope of keeping coal dust out of the respiratory tract.
However, there is little hope of cutting down on the transmission of colds by wearing a mask. Medical authorities believe that most colds spread from nose to hand and then from hand to nose, rather than through the air, so a considerate cold victim should instead wash his or her hands frequently.
The influenza virus does travel easily through the air as a passenger on water droplets, so a mask may head off its transmission.