Why do a lot of Japanese people wear those doctors masks?

I was at new years and I saw some Japanes lady with a doctor mask, in Canada. And I thought about it and remembered seeing a lot of japanese people with those things. What's the deal, is it just sars or do they wear it just because they are afraid of getting something?Why do a lot of Japanese people wear those doctors masks?
It is the cold/flu season. Many commuters in Japan wear these masks during cold/flu season. In the U.S., you'll see individuals wearing these masks during allergy season: while mowing the lawn, yardwork, etc.