A Chipotle Mexican Grill in Massachusetts was temporarily shut down on Wednesday after four employees fell sick after eating their food. One employee has a confirmed case of norovirus, more commonly known as the stomach bug, and two more employees are suspected to also have contracted the illness. The outbreak comes not too long after the chain's most recent series of foodborne illnesses in a nearby Boston outlet that sickened roughly 140 people in December, 2015.

"I don't think it’s the end [for Chipotle]," Lynne Collier, a business analyst at Sterne Agee CRT, told CNBC. "I think it is another negative publicity event that can and probably will impact their traffic. It's a little bit of a setback, but the fact that no one got sick, other than the employees, is a good thing.”

Despite their efforts after the December incident of taking a series of measures to improve food safety procedures, including testing suppliers before they ship, and tracking supplies as they're transported, Chipotle’s stock continues to fall.

In January 2016, the company was hit with a class-action lawsuit that accused the restaurant of attempting to conceal a foodborne illness outbreak before alerting public health officials. The next month, Chipotle restaurants across the nation shut down to conduct a four-hour food and safety meeting with all of its employees after several outbreaks of both E.Coli and the norovirus were linked to its chain.

Norovirus sickens millions of people every year, and thousands end up in the hospital with stomach pain, nausea, diarrhea, and vomiting. In the United States, norovirus is the most common cause of acute gastroenteritis, a condition that inflames the stomach and intestines, which is ultimately responsible for the intense stomach pains.

How The Foodborne Illness Spreads

Anyone can be infected with norovirus, but it can be especially serious for young children and older adults who have weakened immune systems. Each year, the norovirus causes between 19 to 21 million cases of illness and 56,000 to 71,000 hospitalizations, and in severe and untreated cases, 570 to 800 people die.

According to the Centers for Disease Control and Prevention (CDC), the norovirus spreads rapidly. One of the fastest ways is by getting feces or vomit on your hands then putting your fingers in your mouth, or coming in contact with an infected person through shaking their hands, taking care of them while they’re sick, or handling their soiled laundry. Norovirus can live on objects and surfaces for lengths of time, increasing the risk of spread.

However, in the case of Chipotle’s most recent outbreak, it’s more likely caused by contaminated food or water. Most outbreaks occur in restaurants from eating foods or drinks that are contaminated with the virus, or sharing food or eating utensils with someone who is infected with the virus. According to the CDC, infected food workers are frequently the source of the outbreaks from touching raw fruits and vegetables with their bare hands before serving them, however cooked food items can also spread the virus from patron to patron.