Note to anyone concerned about diseases on cruises:
Most cruise ships are so hygienic as to be germophobic -- they take many more measures to prevent the spread of contagious disease than most other 2,000-person environments do -- so, if your wish is for cruise lines to take even more precautions against norovirus, be careful what you wish for!

When I sailed on Celebrity Cruises' Constellation 3 months ago, there were large hand-sanitizer dispensers standing in the middle of hallways (so that everyone would use them upon entering, say, the breakfast room or computer room).  After days in port, when we reboarded the ship, crew members sprayed our hands with sanitizer (without asking first). There were announcements over the P.A. system, and in the ship's daily newsletter, advising passengers to avoid shaking hands with anyone in port (a message antithetical to the whole purpose of travel, dontcha think?). 

My point:  Cruising is an antiseptic form of travel to begin with. Most passengers stay in an hermetically sealed bubble, seeing ports through the windows of tour buses, eating meals onboard instead of on shore, and interacting with few locals other than merchants. Do you really want it to become even more antiseptic than it already is?  You can catch norovirus -- or something worse -- in any large hotel, small town, or doctor's office; you just don't hear about it on the evening news, since an infected cruise ship grabs more headlines.