(CNN) About 170 people who've gone to Yosemite National Park this month are suffering from gastrointestinal illness, including visitors and employees.

Two of the cases are confirmed as norovirus, and the majority of the others are consistent with the virus, the park said in a statement Thursday. Most of the incidents occurred around the first week of January, and there has been a decline of new cases in the past several days, it said.

The park is investigating the circumstances surrounding the illness and is interviewing affected people. It has also enhanced sanitation protocols to prevent further spread of the disease.

Norovirus is the most common cause of acute gastroenteritis -- often called a stomach bug -- in the United States, according to the Centers for Disease Control and Prevention. It causes about 21 million illnesses every year, mostly in crowded environments such as nursing homes and day care centers.

It causes inflammation in the stomach and intestines, leading to stomach pain, nausea and diarrhea. Symptoms include a fever, headache and body aches, and typically last one to three days.

Read More