In Japan, it seems that people wear masks when they are ill as a consideration to others to avoid passing on the illness in a crowded area. Also as a red flag

In Japan, it seems that people wear masks when they are ill as a consideration to others to avoid passing on the illness in a crowded area. Also as a red flag to others, e.g. to the elderly or immune compromised, that they may want to avoid you right now. I frankly don’t mind that. It’s only for a limited time (while you feel ill, or suspect you are coming down with something), it’s very cheap and only mildly annoying. There is the side benefit that it makes you think twice before you go out if you think you might be infectious (is this trip really necessary?).