After 200 park employees and visitors reported bouts of gastrointestinal illness at Yellowstone National Park and nearby Grand Teton National Park this month, national park officials have warned visitors to be vigilant about hygiene.

The outbreak started on June 7, when a group touring the Mammoth Hot Springs complained of stomach flu and other gastrointestinal problems. After the tour group members reported their illnesses, about other 50 visitors and 150 park employees reported similar symptoms.

Preliminary reports found that they had norovirus, or “stomach flu,” which affects up to 21 million people, every year according to the Centers for Disease Control and Prevention.

Al Mash, spokesman for Yellowstone National Park, said campers who were worried about the outbreak should take care to properly store their food and wash their hands with soap and water before eating. “Don’t rely on hand sanitizer. It’s good for a while if you don’t have access to water,” said Mash. “But sanitizer is a poor second to washing your hands.”