Chipotle Mexican Grill stock fell nearly 4 percent Wednesday after an employee at a Massachusetts branch was confirmed to have the highly contagious norovirus. Above, a Chipotle branch in Miami, April 2015. Joe Raedle/Getty Images

Shares of Chipotle Mexican Grill dropped nearly 4 percent Wednesday afternoon after an employee at a suburban Boston branch was confirmed to have norovirus, a highly contagious stomach bug. The restaurant, in Billerica, Massachusetts, closed Tuesday and was scheduled to reopen Thursday in what has become the latest in a series of food outbreaks that have plagued the fast-casual chain.

The popular burrito chain has struggled over the past several months to restore public confidence and repair its reputation after these outbreaks sickened hundreds of customers throughout the United States. More than 50 took ill in two separate E. coli outbreaks that affected 10 states. Locations in Massachusetts and California had outbreaks of norovirus, a stomach bug that can cause vomiting, which affected more than 350 people and were linked to a sick employee.

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Richard Berube, director for the health board in Billerica, Massachusetts, confirmed that one employee at the suburban Boston branch had norovirus and two others were suspected of having the virus. No customers were sickened, according to Chipotle.

The incident comes just one month after Chipotle briefly closed all branches nationwide one afternoon in February for a company-wide meeting on food safety, including a discussion on food-handling procedures to prevent future outbreaks and contamination as well as new sick-leave policies that include not returning to work until an ill employee has been symptom-free for five days.

The outbreaks, which began in Seattle in July, have taken a heavy toll on Chipotle's business. In the fourth quarter of 2015, sales fell by 30 percent compared to the previous year. The company remains under criminal investigation in California for the norovirus outbreak in Simi Valley. The Centers for Disease Control and Prevention said in February that it had wrapped up a separate investigation into the E. coli outbreak without ever determining the cause.

The company said in February it plans to roll out a $50 million campaign that would not address food-safety issues, despite the bad PR the company has suffered of late. Instead, the campaign would “[reinforce] our commitment to high-quality ingredients and great-tasting food,” Mark Crumpacker, the company's chief creative and development officer, said in an earnings call.