Party, norovirus is not a food borne illness. More often referred to as "stomach flu," it's a gastrointestinal illness that is spread from person to person. You get it from someone who has it. If someone gets onto a cruise ship and has the virus, s/he can spread it to countless people, just like spreading a cold. The food concerns center around people who are sick who handle the food -- or the handle! -- and can spread the virus that way.
Because norovirus is contagious, it's often found in places that hold a lot of people, such as schools, jails, nursing homes, hotels, resorts and cruise ships. Cruise ships get a lot of publicity because, unlike resorts or other facilities, cruise lines are obligated to report outbreaks that affect more than 3% of the passengers and crew. Once reported, the press picks up on it and creates a lot of publicity.
The best defense against norovirus is frequent hand washing with soap and water.
You can read more about it here.