Agree with above posters on strict handwashing. And in between try not to touch your face with your hands.
As a hospital nurse I've taught my kids ways of keeping germs at bay and we practice them so faithfully they're now second nature. We use our elbows whenever possible to touch buttons (like the elevator) and either a paper towel or our shirt to open doorknobs everywhere, but particularly in the bathrooms (and don't forget not to touch the toilet handle or door latch inside the stalls, think how gross those are - uugghh - use a piece of clean toilet paper and then throw it in the toilet).
I must say the Wonder was extrememly clean and the measures they take to prevent the spread of germs were impressive. They have signs in the stateroom bathrooms on how to wash your hands effectively, they have kids wash their hands as they enter the club, and outside the buffet they have an (optional) basket of wipes for handwashing before entering/eating. (I asked the CM manning the basket how many people were using the wipes and he said it was a "good response"; however, I observed many walking right by. I also had one problem with the basket routine: the swinging door of the trash can used to dispose the wipes was itself dirty and therefore germy - those washing their hands and then touching the trashcan just defeated the whole purpose of washing their hands! I did report this observation to the dining room supervisor and the dining room "crew chief" but am also planning to report it to
DCL directly - anyone know who I would write to??).
One night DH took DD to the bathroom and observed another Dad whose child had a BM, he helped her wipe, and they both left without washing their hands. Nasty - but just goes to show you have to protect yourself since you can't always count on others to do the right thing.
Lastly, we brought a can of Lysol with us and sprayed the room as much as we could when we first arrived. I then sprayed all bathroom knobs every day even though it was cleaned just because some of the cleaning supplies used by the room steward may have left their own germs.

Many people on the boards talk about bringing sanitizing wipes but I think the spray is more thorough.
I can't imagine anything worse on a cruise than being sick and confined to your cabin.

Two cruises we haven't had a problem.
Olsens, thanks for posting. Again, the flu shot will not prevent a GI bug, it's for a specific respiratory bug.
