Norovirus was to blame for a food-borne illness outbreak last month that sickened nearly 100 customers and employees at Chipotle Mexican Grill in Simi Valley. Seven of the 18 employees who got sick have tested positive for Norovirus, county health officials said Thursday.

The Ventura County Environmental Health Division began investigating after receiving numerous complaints of food poisoning from people who ate Aug. 18-19 at the Chipotle at 1263 Simi Town Center Way. About 80 customers and 18 employees reported symptoms, health officials said.

The county ordered all affected employees to be tested, and those with Norovirus will continue to stay home until further tests show them clear of the virus.

After learning of the food poisoning reports, the restaurant voluntarily closed, threw out all its food, cleaned and disinfected everything, and told all employees with symptoms to stay home, county health officials said.

According to the U.S. Centers for Disease Control and Prevention, those with Norovirus can have symptoms for one to two days, and it can stay in someone’s system for up to two weeks after symptoms disappear.