NEW YORK — Chipotle says one of its stores in Massachusetts is closed after four employees said they were feeling ill.

The closure comes as Chipotle fights to win back customers with stepped up marketing following a series of food scares that sickened customers around the country. Chipotle Mexican Grill Inc. has said it’s instituting a number of food safety procedures to ensure that such incidents don’t happen again. That includes reminding workers they have three paid sick days a year, and that they should stay home if they’re not feeling well.

The Denver-based chain says none of the employees at its store in Billerica, Massachusetts, worked while they were sick, and that no customer illnesses have been connected with the restaurant.

A woman who answered the phone for the Billerica Board of Health office declined to comment.

A director for the city board of health told USA Today the store closed Tuesday after an employee tested positive for norovirus. A Chipotle spokesman did not respond when asked to confirm that an employee tested positive for norovirus, or when the store is expected to reopen.

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 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 $24.69, or 4.7 percent, to $500 in midday trading Wednesday. Its shares are down more than 24 percent over the past year.