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A school district in Colorado has closed more than 40 schools for two days after a virus outbreak rapidly spread through the student population, according to school officials.

It’s the first time the Mesa County Valley School District 51 has had to close all schools due to illness, the district said Wednesday.

“We are taking this highly unusual action because this virus is extremely contagious and spreading quickly across our schools,” Nursing Coordinator Tanya Marvin said in the statement.

More than a dozen schools in the state’s 14th largest district reported increased absences “due to illness and several incidences of vomiting in public areas of the schools,” according to the Mesa County Public Health Department.

A second, related virus has also been affecting students in recent weeks, the district said. “The combination of the two has created an unprecedented spread of illness.”

“Onset of symptoms for both types of viruses, including vomiting, is incredibly fast. The second version also causes fever in several cases,” the district statement said.

The health department says it is working to identify the illness, which is “acting a lot like norovirus” and lasts between 12-24 hours.

Norovirus, sometimes called the “stomach bug,” is easily spread through direct contact, consuming contaminated food or water, or touching contaminated surfaces and then putting your hands in your mouth, according to the Centers for Disease Control and Prevention.

The closure includes all after-school activities, the district said, and schools will remained closed until after Thanksgiving break.

Mesa County Valley School District 51 is the largest school district between Salt Lake City, Utah, and Denver, Colorado, the district website says. It serves more than 22,000 students in 46 schools and programs, employing nearly 3,000 employees.