Wash your hands - wash your hands - wash your hands.
I can't count the number of times I have seen women/children use the bathroom and walk directly out the door without washing their hands. It's disgusting. I will not use the public washrooms on a ship if I can get back to my cabin first. I've been told by my husband it's worse in the men's washrooms.
Apparently the CDC requires a report when 3% of the shipboard complement are diagnosised with Noro. Of course there are always a lot who won't self report because they don't want to be quaranteed.
Usually, if you are careful, you should be OK. Wash your hands, avoid places where people are coughing etc, use a sleeve, kleenex or whatever to push elevator buttons, be mindful of the handrails when going up and down the stairs, don't touch food that has been "manhandled" ahead of you.