(Reuters) - Eighty Boston College students fell ill after eating at a Chipotle Mexican Grill CMG.N restaurant this past weekend, and early test results point to the highly contagious norovirus as the culprit, public health investigators said on Tuesday.

Customers enter a Chipotle restaurant in Seattle, Washington November 20, 2015. REUTERS/David Ryder

Chipotle, already grappling with a multistate E. coli outbreak that has battered the burrito chain’s sales and stock price, has temporarily closed the Cleveland Circle restaurant where the Boston College students reported eating.

The students are being tested for E. coli and norovirus, which both can cause severe vomiting and diarrhea. Those test results are not expected for at least two days, a university spokesman said.

Chipotle on Tuesday said it would not shutter any other outlets in the Boston area.

“The pattern here looks like norovirus isolated to one restaurant,” Chipotle spokesman Chris Arnold told Reuters.

The chain has been under a microscope since Oct. 31, when it was first linked to an E. coli outbreak that has sickened 52 people in nine states.

That outbreak was the company’s third food safety incident since August. It has raised concerns about potential reputational damage to the fast-growing brand that has won a loyal following for its food made with fresh produce, meats raised without antibiotics and ingredients that are free of genetically modified organisms, or GMOs.

In August, Norovirus was blamed for sickening nearly 100 people at a Chipotle restaurant in Simi Valley, California.

Norovirus is the most common cause of foodborne-disease outbreaks in the United States. It causes as many as 21 million illnesses annually, often in places such as hospitals, cruise ships and universities where people eat and live in close quarters.

Norovirus can persist in an environment for up to six weeks, said Benjamin Chapman, a food safety specialist at North Carolina State University.

“Every time you have a vomit event, you’re looking at billions of (virus) particles, and it takes only a few to make you sick,” said Chapman, who said it was too early to say whether the students got norovirus at Chipotle.

Stephen Anderson, an analyst at investment bank and wealth manager Maxim Group, said the Boston illnesses are a “near-term negative reinforcement” when viewed along with the E. coli outbreak and other health-related issues Chipotle has dealt with since the summer.

Chipotle’s shares, which closed down 1.7 percent at $542.24, moved marginally higher in extended trading on the norovirus test results.