(Reuters) – Some 170 people who have spent time in Yosemite National Park in recent weeks have suffered from a gastrointestinal ailment “consistent with norovirus” and two have been diagnosed with the illness, park officials said on Thursday.

Most of those who became ill spent time in Yosemite Valley during or around the first week in January, park spokesman Scott Gediman said in a written statement, while the number of new cases reported has declined in the past several days.

Yosemite and national park health officials were investigating the outbreak, Gediman said, adding: “The overwhelming majority of the reported cases are consistent with norovirus.”

The U.S. Centers for Disease Control and Prevention describes norovirus as a very contagious stomach illness, spread by contact with an infected person or contaminated surfaces, that causes vomiting and diarrhea.

Gediman said Yosemite was undertaking “extensive cleaning and enhanced sanitation protocols” following the outbreak.

(Reporting by Dan Whitcomb in Culver City, California; editing by Richard Pullin)