You can't catch the norovirus from breathing the same air as an infected person. You have to somehow get fecal matter (v or d) in your mouth for it to be swallowed and enter into your intestines where it will then cause distress. The only way you would catch it breathing it, is if you were standing directly next to someone who was v-ing, and some of the v droplets got into your nose or mouth that were floating in the surrounding air. If you keep your hands away from your face, especially your eyes and mouth, and wash them thoroughly before you touch anything that goes into your mouth, you decrease your chances of getting the stomach virus by a lot. It's spread through fecal-oral contact so don't worry about being in a crowded place with someone who has it. Not possible.