Yellowstone and Grand Teton National Parks are fighting an outbreak of norovirus, the highly contagious gastrointestinal scourge of cruise ships. According to Park officials, the virus has already left 150 employees and 50 guests sick.

In an announcement this week, the National Park Service (NPS) say 200 people have been stricken with norovirus, a nasty gastrointestinal illness that is highly contagious. Those sickened were visiting the park in northwestern Wyoming as well as areas in Montana outside the two parks, CNN reports.

The warning advises park visitors to wash their hands thoroughly to help keep the virus from spreading. The warning also requires businesses to increase cleaning and disinfection of all public areas and also asks potentially infected park workers to isolate themselves until they are symptom-free of the virus for at least 72 hours.

"Given the nature of the illness, having some cases confirmed prompts us to treat any reported illness with similar symptoms as if it were a confirmed case of norovirus," Yellowstone spokesman Al Nash said in a statement to Jackson Hole News.

Officials at Yellowstone said the outbreak of norovirus began June 7, when several people on a tour bus complained about feeling ill.

"It's been almost a decade since we've seen anything on this scale here," said Al Nash, a Yellowstone spokesman, New York Times notes.

However, this isn't the first time that illnesses have plagued national parks out west.

Last summer, at least eight visitors to Yosemite National Park contracted hantavirus pulmonary syndrome. Three of them died.