In a statement issued Wednesday, Tanya Marvin, the school system's nursing coordinator, said the "highly unusual action" is being taken because of the outbreak's rapid spread among the district's roughly 22,000 students.
"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," Marvin said in the statement.
Although public health officials have not identified the illness officially as a norovirus, Jeff Kuhr, the executive director of Mesa County Public Health, told The New York Times, reported symptoms of vomiting and diarrhea usually lasting between 12 and 24 hours are consistent with that diagnosis.
In addition, district spokeswoman Emily Shockley told the Times that every school that reported more than a 10 percent absentee rate prior to the announced closure ha been wiped down, bleached and sprayed with disinfectant.
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