I have a fairly reliable immune system. In spite of this, I’ve found myself wearing a humidiflyer mask on plane journeys in the past. The panicked look from the air-hostess as you put on the mask could be enough to make one avoid making a habit of this.
Singers are fanatical about avoiding catching colds and will do anything to avoid being coughed or sneezed on, including running out of shops if someone sneezes or just walking off fast if someone with a hint of a sniff starts to talk to you.
For added protection, tenors wear scarves.