DENVER — David Breskin was in line for the breakfast buffet at the Old Faithful Inn at Yellowstone National Park this week when he noticed something unusual: hotel employees in gloves were handing out plates and serving eggs and French toast.

When Mr. Breskin asked what was going on, an employee said, “Oh, it’s because of the virus.”

Near the peak of the summertime 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 that 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.

On Wednesday, the National Park Service issued a statement 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.