Norwalk Virus / Norovirus
Norwalk Virus, also known as Norovirus, is a common stomach virus. The reason it spreads so easily on cruise ships is that the people are all stuck together for several days. It's not that the cruise ship is "bad" or "dirty" when this happens. It's more like a person with chicken pox goes into a sealed room with 20 people, and they all get it. It's just what happens.
Like most virusses, Norovirus is spread by one person being infected, and then sneezing out germs. The infected person may also touch their mouth for some reason and then touch a doorknob or railing. Now another person who walks along, touches that item and then touches their mouth will introduce the virus into their own system.
That's the whole trick - your skin knows to block out virusses. So as long as you keep things that go near your mouth clean, you should be all set. So any time you are preparing to eat, wash your hands thoroughly. Any time you use the restrooms, wash your hands. Keep your hands away from your mouth - i.e. try not to chew your nails.
Yes, there will always be sick people on a cruise. There are always sick people in schools and offices, too! It's up to you to take normal precautions against getting sick yourself. The virus won't "inject" itself into your body :) As long as you help prevent the normal pathways of infection, you should be fine.
You can of course also take along multivitamins and vitamin C, just to keep your immune system healthy and working.
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