More than 600 students in Utah stayed home from school on Thursday over concerns about a potential norovirus outbreak.

Health officials had warned on Wednesday that several cases had been reported in the Alpine School District, in Utah County, while the health department said it received reports of students elsewhere vomiting and experiencing diarrhea.

Letters about the illnesses sent home to parents urged them to keep students who feel ill home for 72 hours after symptoms have ended. Students with a sibling experiencing symptoms were also asked to stay home.

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“This letter is to inform you that an outbreak of gastrointestinal illness has occurred in several schools nearby the school your child attends,” the Utah County Health Department letter said, according to Fox 13 Now. “Because this illness has spread so quickly, it appears highly contagious and may likely enter your school.”

According to the Deseret News, parents calling in to report their child absent have been asked to say if their child has experienced potential symptoms of norovirus. Officials also noted that it’s entirely possible that cases have gone unreported, with students headed back to school easily spreading it amongst themselves.

“Just getting back to school, it’s always a little bit hard because you get kids in more of a confined space together,” Aislynn Hill, Utah County Health Department spokesperson, told Desert News. “It’s entirely possible that there have been others that we’re just not aware of.”

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Norovirus is a highly contagious virus that can infect anyone through direct contact with an infected person, consuming contaminated food or water or touching contaminated surfaces and then putting your unwashed hands in your mouth. The illness can cause diarrhea, vomiting, nausea and stomach pain.

In order to prevent the virus from spreading, the Centers for Disease Control and Prevention (CDC) recommends washing hands often, rinsing fruits and vegetables, cooking shellfish thoroughly, staying home when sick and avoid preparing foods for others while sick and for two days after symptoms stop.