A Colorado high school was shut down after 300 students and staff members were struck by a mysterious illness.

According to the Mesa County Valley School District 51, Palisade High School closed on Thursday morning at 10:30 a.m. due to a strange vomiting illness. According to Colorado Public Radio, the spread of the mysterious illness more than doubled in size over the last two days, climbing from 130 cases on Wednesday to 300 cases on Thursday. Palisade has a staff of 45 and a student body of approximately 1,000.

The Mesa County Valley School District 51 issued a statement Thursday morning stating, “student and staff health and safety is always a priority, and we feel that closing the school for cleaning, and to allow students and staff to rest and not continue to spread the illness, is in the best interest of the entire school staff and student population.”

The closure will remain in effect on Friday, November 15, allowing for deep cleaning of the campus to help minimize any future spread of the unknown illness. The school is expected to reopen by Monday, November 18.

The specific type and cause of the illness have not been confirmed yet.

The school advised that all students who have vomited more than twice in the last 24 hours or if vomit appears green or bloody should remain home. Students were also told to stay home if they’ve experienced a fever, diarrhea, or a recent head injury.

This story is developing.