A mysterious virus ransacked a Massachusetts school on Friday, leaving nearly half of its student body sick and forcing the school to close. Groton-Dunstable Regional School District Superintendent Laura Chesson said 200 of 530 students at Florence Roche Elementary School in Groton fell ill with the virus by Friday morning, according to WCVB. Due to the sudden and quick spread of the virus, along with the lack of contaminants in the school’s food and water, the district believes it was likely norovirus. “We did deep cleaning, disinfecting,” Chesson said. “We had checked food, milk and water supply and all of those things were deemed appropriate.” Norovirus symptoms include vomiting, diarrhea, and stomach pain, among others. Chesson said the school expects to see students return—and healthy—on Monday.