This may be a virus; however, one cruise we were on over 100 people sick. I was but hubby wasn't and we shared the same bed etc. Ship said it was norvo but I think it was bad mussels. My own survey found most of us ill became so right after dinner and all had the mussels. This happened at every seating. I think cruise lines are quick to say Norvo so they can say passengers brought it aboard instead of we served you shellfish from warmed water. Which, by the way, I read later that lots of shellfish had been taken near Thiland in waters that were too warm.