Near the peak of the summer tourist season, Yellowstone and Grand Teton National Parks are fighting an outbreak of norovirus, the highly contagious gastrointestinal scourge of cruise ships. Park officials said the virus had so far sickened 150 employees and 50 guests.

Employees at the parks' lodges and restaurants have been scrubbing rooms and sanitizing every surface to try to stop the spread of the virus. Sick employees, and some who might have been exposed, have been quarantined.

The National Park Service is warning visitors to the two parks to wash their hands to avoid the virus, which sickens about 21 million people every year in the United States.

So far, park officials said that only a small percentage of visitors had fallen ill. Yellowstone draws more than three million visitors every year, and about 2.5 million people visit Grand Teton.

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