Norovirus has been identified as the culprit behind an outbreak of stomach flu at the Sunset District’s St. Ignatius College Preparatory.

“This doesn’t change anything that we are doing here at school to prevent illnesses from spreading, and the chemicals that we have used to clean our facility will eliminate noroviruses,” Principal Patrick Ruff said in a statement posted on the St. Ignatius website.

Noroviruses are a group of related viruses that can spread quickly, and swiftly cause symptoms including vomiting, diarrhea and stomach cramps. The federal Centers for Disease Control and Prevention agency says that noroviruses are the leading cause of food-borne illness in the United States, but the water supply and food-service areas at St. Ignatius reportedly have been tested and ruled out as the pathway for the outbreak.

St. Ignatius has been closed since Wednesday because hundreds of students at the 1,444-student school have been sickened. Classes are set to resume Monday.