That's because people who've had the virus can still be contagious after they feel better.

The virus is more common in winter and is usually difficult to avoid. But doctors say the best protection is to wash your hands often, and wash them well.

Edmonton is also in the midst of an outbreak, with 750 cases confirmed so far at the city's schools, hospitals, daycare and nursing homes.

Decontaminating surfaces is one solution, but cleaning won't keep it at bay.

Tim Gallagher

"The ship last Friday was perfectly sanitized," said Tim Gallagher of Carnival Corp. If "someone comes on board with the virus, any efforts that you had up to that time is for naught."

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