Check out the Travel Channels new show, Travel Advisor, Safety at Sea. They had different segments, on missing passengers,virus's onboard, sexual attacks,and fires on ships, specifically, the Star Princess
One thing I found interesting, that I had not heard before was, they interviewed a couple, who both came down with the noro virus, on the second day of the cruise. They went to the medical facility,and were given medication, and told they needed to be quarentined for 24 hours. It seemed to work well, so they decided to get off at port the next day. When they exited the ship, their sign and sail cards had been deactivated, and were directed back to their cabin, with instructions they had to remain in there for 72 hrs. This was contrary to what they had first been told.
So, I guess the fact they went to the dr. was the trigger for de-activation. I'm not saying this is wrong to do, for the protection of a mass breakout, I was just never heard about this action being taken.
If you can catch this show in reruns, watch it, it was interesting.