Thousands of students in Colorado were out of school Thursday because of a suspected outbreak of norovirus.

All 46 schools in the district serving Grand Junction and the surrounding area were closed and won’t reopen until after the Thanksgiving break so custodians can disinfect surfaces.

OTC Editor’s Note: The district calendar also states that no elementary or middle school will be in session on December 2nd, though high school is set to resume at this time.

Norovirus is a highly contagious bug that causes diarrhea, vomiting and other symptoms, and can spread quickly in public places. It’s most common in winter but can be seen any time of year.

The outbreak was first seen last week at a high school before spreading to other schools.

Officials in the district, the largest in western Colorado with 22,000 students, believe there is now another related strain of the virus that has affected some students who were previously sick.

“We are taking this highly unusual action because this virus is extremely contagious and spreading quickly across our schools,” district nursing coordinator Tanya Martin said.

It’s not clear how many students have become sick. Some custodians also became ill, slowing down the cleaning process, but it’s unknown if the suspected viruses were to blame.

Nicole Comstock, communicable disease deputy branch chief for the state Department of Public Health and Environment, told The Daily Sentinel that there are about 150 to 200 suspected norovirus outbreaks in the state each year.

She said most occur at nursing homes but outbreaks at schools are not uncommon.

The U.S. Air Force Academy outside Colorado Springs reported that 400 cadets have been sickened by norovirus since October, but the outbreak hasn’t disrupted classes.

The Gazette reported that infected cadets are kept on bed rest for at least three days after their symptoms disappear before they can be cleared to return to class and training.