Two of America's premier national parks, Yellowstone and Grand Teton, warned visitors on Wednesday about a gastrointestinal illness that has sickened at least 200 people at the start of the summer tourist season.
The rare health advisory, tied to a suspected outbreak of the highly contagious norovirus, comes in the early weeks of a season that drew about 6 million people to the parks last year.
The warning advises visitors to the parks in northwestern Wyoming to wash their hands to stem the spread of the virus, which causes a flu-like ailment whose symptoms include stomach pain and vomiting, park officials said.
They have required businesses such as restaurants and lodging facilities to increase cleaning and disinfection of all public areas and have asked potentially infected park workers to isolate themselves until they have been symptom-free for at least 72 hours.
A tour group visiting Yellowstone, home to the Old Faithful geyser, first complained June 7 of symptoms linked to norovirus, the most common cause of acute gastroenteritis in the United States, according to the federal Centers for Disease Control and Prevention.
The illness has since affected 100 Yellowstone employees, 50 Grand Teton workers and at least 50 visitors, park officials said.
The warning comes a year after Yosemite National Park last September warned 230,000 visitors of a hantavirus outbreak that had infected nine overnight visitors and killed three.