Jan. 17 (UPI) -- Yosemite National Park said some 170 people who visited the park this month have fallen ill with gastrointestinal illness.
The California park said in a statement Thursday the illness has affected both visitors and employees, most of who spent time in Yosemite Valley during the first week of the year.
Two cases have been confirmed to be norovirus, which the U.S. Centers for Disease Control and Prevention says is a very contagious virus that causes diarrhea, vomiting, nausea and stomach pain.
"Outbreaks can happen anytime, but they occur most often from November to April," the CDC said.
The park said officials and medical professionals with the National Park Service Office of Public Health are investigating the cause of the illness and conducting interviews with those affected.
"The overwhelming majority of the reported cases are consistent with norovirus," it said.
To prevent infection, the park is encouraging visitors to wash their hands frequently with soap and water and if soap and water are not available, use an alcohol-based hand sanitizer that contains at least 60 percent alcohol.
"If you do experience any illness, please stay hydrated and contact your healthcare provider if you have any concerns," the park said. "Please limit your interaction with other people as much as possible to prevent further spread."