The National Park Service reports that at least 150 cases of the norovirus have struck employees and visitors in popular national parks.

People visiting certain U.S. national parks should be vigilant about washing their hands due a rise in cases of the highly contagious norovirus, according to the National Park Service.

There have been more than 100 cases of stomach flu related to the norovirus in Yellowstone National Park, which is located in portions of Idaho, Montana, and Wyoming, and about 50 suspected cases in the Grand Tetons. The infections involved both park visitors and employees, the park service said.

Cases began appearing in Yellowstone on June 7 when a tour group at Mammoth Hot Springs showed signs of stomach flu and other gastrointestinal discomfort.

The norovirus is the most common cause of acute gastroenteritis in the U.S., causing 21 million illnesses each year, resulting in 70,000 hospitalizations and 800 deaths, according to the U.S. Centers for Disease Control and Prevention (CDC).

Because of the high volume of visitors to the nation’s 401 national parks, there’s more opportunity for viruses like these to spread.

To combat further problems, employees of the park service and nearby businesses have been vigilant in cleaning and disinfecting public areas, including restrooms and store counters.

Employees who may have been infected have been isolated until they are free of symptoms for at least 72 hours, the park service said.