NEW YORK (AP)--A Chipotle store in Massachusetts was closed Wednesday amid concerns some employees had contracted the norovirus.

None of the sick employees came in to work, but the store was closed as a precautionary measure, said Richard Berube, director for the health board in Billerica, Massachusetts.

Berube initially said his agency confirmed one case of norovirus and found two other suspected cases. But he later said in a news release there were three suspected cases. He did not return email and phone messages Wednesday night seeking a clarification.

Chipotle first closed the store Tuesday at the board's suggestion, and it is scheduled to reopen Thursday, he said.

Chipotle is already fighting to win back customers following a rash of incidents in which customers were sickened around the country. Chipotle Mexican Grill Inc. has said it's instituting a number of food safety procedures to ensure customer safety. That includes reminding workers they have three paid sick days a year, and that they should stay home if they're not feeling well.

Four employees reported feeling ill at the store in Billerica, Chipotle said. None of them worked while they were sick and no customer illnesses have been connected with the restaurant, according to the Denver chain. It said the store received a "perfect score" when it was inspected by the health department on March 3, but that it nevertheless closed the store for a "full sanitation."

The illnesses were reported by the health facility to the state health department, Berube said, which then notified the town board of health.

While closing restaurants due to illnesses is rare, Berube believes there are "other cases here and there that fly below the radar."

According to the Centers for Disease Control and Prevention, norovirus is very contagious and the most common cause of foodborne outbreaks in the United States. It says that infected workers are frequently the source of outbreaks and that the virus causes 19 to 21 million illnesses a year.

During a national meeting for workers in February, Chipotle reminded employees that two of the company's four recent food scares were the result of norovirus.

"If you're feeling sick, especially if you've vomited, whether at work or at home, you need to let your manager or your field leader know right away," co-CEO Monty Moran said during the webcast.

Employees were told to watch for symptoms such as nausea, vomiting, explosive diarrhea, yellowing of the skin and eyes and dark urine.

Chipotle shares fell $18.06, or 3.4 percent, to $506.63 Wednesday. Its shares are down 23 percent over the past year.

--Associated Press