Normally walking into a bank with a mask on might trigger panic, but in Japan it barely warrants a second glance. They are becoming more evident in Canada, but Japanese people have been using disposable masks for many years. Originally I thought they were being worn to protect those around them, but it appears the reasons are usually not altruistic.
Yayoi and I have not listened to "big sister's" advice to wear them and in Osaka we visited the aquarium on a busy Sunday when many kids were there with their parents. On the bus ride to Osaka I felt a little funny and sure enough in Takamatsu I was quite sick for a few days and Yayoi for one day.