A highly contagious gastrointestinal illness has swept through Yosemite National Park, sickening at least 170 people and prompting a huge cleanup effort at the hotels, restaurants and public facilities in the picturesque valley, federal officials said Thursday.

The illness, characterized by stomach issues, including vomiting and diarrhea, has stricken park visitors and employees in every part of Yosemite Valley, with most cases reported during the first week of January, park spokesman Scott Gediman said.

Two cases have been confirmed as norovirus, an extremely contagious virus that is notorious for sickening people on cruise ships, in schools and other crowded places, a National Park Service release stated.

It isn’t known whether the other cases were caused by the same virus, but “the overwhelming majority of the reported cases are consistent with norovirus,” the release continued.

“It’s highly contagious, so if an infected person touches a banister and someone else touches it, they can get it,” Gediman said. “It’s very difficult to find out exactly where it started.”

Norovirus, which causes as many as 21 million cases of acute gastroenteritis in the United States every year, can spread through direct contact with an infected person, touching a contaminated surface or object, or by eating food or drinking liquids contaminated by someone with the virus. The virus can also spread through airborne transmission. It can take 12 to 48 hours before symptoms appear.

Michael Bennett, 69, and his partner, Luke Lowery, 60, spent two nights at the famed Ahwahnee Hotel and on Jan. 4, took a train home to San Francisco, where Lowery got sick.

“That night he was violently ill,” Bennett said. The next day “I began feeling queasy and by that night I also had severe vomiting and diarrhea.”

The symptoms lasted about three days. Bennett said they spent most of their time at the Ahwahnee, where they ate most of their meals. But they did take the park shuttle, went into shops and museums and ate one meal at the Yosemite Lodge.

“I’m upset because we love going to the park, and we love going to the Ahwahnee, but now we are going to feel a little bit wary,” Bennett said. “It’s kind of disgusting to think that somebody who didn’t wash their hands may have passed on the virus.”

Kathleen Morse and her husband, Jeremy Zawodny, got sick about 24 hours after celebrating their 12th anniversary by spending the night of Jan. 7 at the Ahwahnee.

Morse, the 45-year-old owner of an aerospace research company in Groveland (Tuolumne County), said they, too, spent most of their time at the famous hotel, but also had a lunch at Yosemite Lodge. She was struck by the fact that there was no hand sanitizer in the restrooms.

Preventing transmission of norovirus Wash your hands frequently with soap and water for at least 20 seconds. If soap and water are not readily available, an alcohol-based hand sanitizer that contains at least 60% alcohol can be used. Avoid sharing food or drinks with other people, especially when you’re sick or they’re sick.

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“I think that there’s a lot of people going through those areas,” said Morse, who was mostly upset because nobody told her anyone else had gotten sick and she had to call three times before a hotel manager took her report. “They can’t cut corners.”

The Yosemite health clinic staff is working with specialists with the U.S. Public Health Service and the California Department of Public Health interviewing people and conducting inspections of the many snack shops, restaurants, hotels and other facilities operated by Yosemite’s vendor, Aramark of Philadelphia.

Tracking down the cause of the epidemic will be difficult, Gediman said, so Park Service staff are concentrating on preventing its spread by disinfecting hotel rooms, restaurants and other public places.

“The good news is that most of the cases were around the first of the year, and we’ve seen a significant decrease,” he said. “We’ve had very few new cases reported lately, so we’re hopeful that we’re on the downward cycle of it.”

The incident comes as Yosemite and Aramark struggle to maintain adequate visitor services at the park. Aramark, which has managed Yosemite’s concessions for nearly four years, has been repeatedly criticized for poor food quality, inadequate shuttle service and other problems.

The Chronicle recently reported that the Ahwahnee Hotel has been downgraded from its prestigious four-diamond rating by AAA to three diamonds.

It is the second major virus to hit Yosemite since 2012, when a deadly outbreak of hantavirus sickened nine people, three of whom died.

The virus was blamed on a skyrocketing deer mice population in Yosemite Valley. The victims in that case are believed to have inhaled airborne particles of mouse feces and urine that accumulated in dust.

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com Twitter: @pfimrite