You catch it by eating or drinking contaminated food or drink that has vomit particles in it. Just wash your hands after the rest room, and before eating or drinking, and you should be fine. Also, never put your hands in your mouth! Norwalk virus is not airborne, so you can really only catch it if your hands are contaminated, and then you put your hands in your mouth. Also, use a bleach cleaner to clean kitchens and bathrooms, and make sure to avoid physical contact with a sick person. If you never put your hands in your mouth, you will most likely never get sick.