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.

While we retreat to the outdoors to escape the noise and pollution of the city, humans bring bits and pieces of themselves into the woods, especially the often crowded campgrounds at public parks.

The CDC recommends frequent hand washing to reduce your risk of contracting the norovirus and other viruses that can cause stomach upset. Alcohol-based hand sanitizers are a good short-term remedy, but soap and water should be used after coming in contact with potentially contaminated surfaces, such as bathrooms, cooking utensils, and food.

Besides protecting yourself from germs and bacteria that can wreak havoc inside you, you should also take precautions against something much bigger: the sun. Wearing protective clothing, regularly applying sunscreen, and finding shade during the warmest parts of the day can prevent serious problems like sunburn and sunstroke.

If you have other concerns, ask a park ranger about the safety precautions you and your family should take.