The first official norovirus outbreak on a cruise ship this year, according to Jim Walker of Cruise Law News, involved the Norwegian Cruise Line (NCL) Norwegian Star.
The outbreak occurred during a cruise from January 5-19, 2014. The virus sickened 130 of 2318 cruise passenger (5.61%) and 12 of 1039 crew members (1.15%). You can read the CDC report here.
The CDC concluded that the virus in question which sickened the 142 or so people was norovirus. This was the “causative factor” in CDC parlance. The CDC can usually figure out the “causative factor” and most of the time norovirus is the culprit. But I have never seen a CDC report in the last 10 or 15 years where the CDC figured out how the norovirus came aboard the cruise ship.
The cruise lines always blame the passengers. Sometimes the blame is direct with a cruise line public relations representative pointing the finger at their guests. Sometimes it is more subtle with no blame assessment but in the form of “passengers-need-to-wash-their-hands” type of admonishment.
Determining the cause of a norovirus outbreak is a scientific process to be made by epidemiologists and doctors, not cruise line PR people.