Colorado School District Closes Due to ‘Extremely Contagious’ Virus Spreading at ‘Unprecedented’ Rate

Over 40 schools in Colorado have been closed this week to try and prevent the spread of an as-of-yet unidentified virus, which school officials have labeled as “extremely contagious.”

Colorado’s Mesa County Valley School District 51 — which is the 14th largest in the state, serving over 22,000 students — announced the closure on Wednesday, noting that all schools will remain closed until after Thanksgiving Break.

“The health and safety of our students and staff is our top priority,” read a statement from Dr. Diana Sirko, Superintendent of Schools for the district, which also noted that the decision to enforce a “districtwide closure” was unprecedented.

“We are taking this highly unusual action because this virus is extremely contagious and spreading quickly across our schools,” said D51 Nursing Coordinator Tanya Marvin. “In addition, it appears that there is now a second, related virus that is affecting students, some of whom have already been ill in recent weeks. The combination of the two has created an unprecedented spread of illness.”

According to the statement, the onset of symptoms, which include vomiting and fever, “is incredibly fast.”

Although the school has yet to publicly identify the virus, the Mesa County Public Health department issued a separate statement which noted its similarities to norovirus.

“This illness although not identified, lasts between 12-24 hours, and is acting a lot like norovirus,” read the statement, which added that “determining an exact diagnosis for these types of illnesses requires laboratory tests.”

Norovirus causes inflammation of the stomach or intestines or both, according to the U.S. Centers for Disease Control and Prevention, and symptoms — which include diarrhea, vomiting, nausea, stomach pain and sometimes fever, headaches and body aches — typically develop within 12 to 48 hours of exposure.

Because the bug is extremely contagious, it can spread easily in places like schools, daycare centers, nursing homes and cruise ships.

You can become infected through contact with stool or vomit of infected people. This can happen through eating food or drinking liquids that are contaminated, touching surfaces or objects containing with the virus, or having contact with someone who is infected.

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“The decision to close is the right move,” added Mesa County Public Health Executive Director Jeff Kuhr in the 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. This will give those buildings a window to disinfect and start fresh after the Thanksgiving holiday.”

The statement went on to note that the decision to close all of the schools was made “after more than a dozen schools reported increased absenteeism due to illness and several incidences of vomiting in public areas of schools.”

School district spokesperson Emily Shockley told TIME that over 3,400 students were absent from school this week, prior to the closure.

In a Facebook post earlier this week, the school district announced that “about a dozen” schools, all of which had an absenteeism rate of at least 10 percent, had “received extra cleaning.” The rest of the schools will also receive “extra disinfecting” during the closure.

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On Thursday, another nearby school district also announced that all of their schools would close on Friday, reopening after Thanksgiving Break.

In a statement, the Superintendent of Schools for the West Grand School District said they were experiencing a “sharp rise” in health issues, “most related to a gastrointestinal issue or an upper respiratory issue.”