I don't know if it is still running around cruise ships, but I had it two Thanksgivings ago....not in anyway related to cruise ship or having recently been on a cruise ship. I was really, really sick for 48 hours. Couldn't move too far away from a trash can or bathroom. Gatorade and phenegran helped. Can't imagine having that on a cruise........
Of the 10 million folks who cruised out of North American ports in 2004 the outbreaks of Norwalk virus ( or any other stomach flu type virus) hit such a small percentage of folks it is hard to measure. When it does happen, because of the close proximity of folks on the ship sharing the same spaces etc, it can spread . Outbreaks can impact
several dozen to several hundred folks at a time. Ships react swiftly to combat the problem. It is no more prevelent on cruise ships than it is in nursing homes , schools or offices. The main difference is that when it hits a school, kids are kept home.
When it hits a cruise ship, someone from the 6 oclock news meets the ship with a camera crew. Practice good hygiene regarding hand washing frequently etc. to avoid problems (this is a good common sense practice anyway).
I personally do not consider this an issue when travelling.