A violent stomach bug has exploded in Colorado’s Mesa County Valley School District 51, leading to the swift shutdown of the entire district, which includes 46 schools and programs that serve more than 22,000 students.

The outbreak first struck a high school, which was closed down Thursday, November 14. In the subsequent days, the outbreak took out five other schools and led to more than 5,000 absences in others (due to either illness or fear of the outbreak). On Wednesday, November 20, district officials made the bold decision to close down the entire district through the Thanksgiving holiday.

Health officials have not officially identified the pathogen causing the outbreak but suspect it is norovirus—a highly contagious germ that causes vomiting and diarrhea, typically for one to three days. Norovirus is the leading cause of such stomach illnesses in the US.

"The decision to close is the right move,” Mesa County Public Health Executive Director Jeff Kuhr said in a statement. “Past experience with these types of viruses tell us having a period of time away from close person-to-person contact can be instrumental in these illnesses running their course.”

The district will use the time to try to disinfect its facilities—a particularly odious and difficult task when it comes to a norovirus outbreak.

Kuhr told The New York Times that many sick students had been vomiting on the floors of classrooms and hallways, unable to make it to trash cans or bathrooms in time.

“It was pretty out of control,” Kuhr said. “If all the stars align, norovirus could really wreak havoc on a community. It just feels like that right now.”

Norovirus victims can spew billions of infectious viral particles in their vomit and feces. Just a few can cause a new infection. Kuhr estimated that everything within a 25-foot radius of a puking was likely contaminated.

The Associated Press reported that some school custodians also fell ill, slowing the disinfection process.

County officials are telling residents to wash their hands frequently and stay home if they feel ill.