A Chipotle Mexican Grill restaurant is seen in Washington, D.C., Dec. 22, 2015. Getty Images

Chipotle Mexican Grill on Tuesday said the latest norovirus outbreak in one of its restaurants in Virginia last week and the subsequent $1 billion decline in the value of the chain over a period of five days was caused due to an employee coming to work sick and a lax enforcement of the company’s sick policy.

"We conducted a thorough investigation and it revealed that our leadership didn’t follow our protocols," said Chipotle founder and Chief Executive Steve Ells during a second-quarter earnings conference call on Tuesday, according to New York Post.

"We believe someone was working while sick," Ells said.

Read: Chipotle Mexican Grill Reports First-Ever Quarterly Loss As Same-Store Sales Plunge In Wake Of Food Safety Crisis

The sick staffer was believed to have spread the norovirus around a suburban Washington D.C. store, which led over 135 customers to fall sick after visiting the restaurant.

More than 135 people reported falling sick between July 13 and July 16 after visiting the burrito chain’s restaurant in Sterling, Virginia. The Loudoun County Health Department stated Monday its investigation into the reports found two customers tested positive for the same strain of the virus after reports of the norovirus from the beginning of this month.

"Two ill patrons have tested positive for the same strain of norovirus. Based on symptoms reported and these preliminary laboratory results, the cause of the outbreak is believed to be norovirus, though the specific source of the norovirus has not yet been identified," Dr. David Goodfriend, director of the Loudoun County Health Department said in a statement.

"The Health Department is not aware of any customers becoming ill since the reopening of the facility last Wednesday," Goodfriend added.

After Chipotle announced one of their employees came to work sick and spread the virus, Director Goodfriend said given "how quickly symptoms develop with norovirus and the large number of folks involved, we will likely not be able to determine whether a patron or employee was the first to introduce the virus to the facility," affiliated to USA Today network reported.

Read: Will Chipotle Have Another Outbreak? Food Safety Advocates Plan Protest Amid Concerns

Last week after the news of the outbreak, the company faced another hit when a video of rodents running around a Dallas Chipotle restaurant went viral, due to which Chipotle’s shares went down by 13 percent last week, cutting more $1 billion off the burrito chain’s market capital. The recent outbreak of the norovirus and the disclosure of the rodents video was a huge setback for the company as it was already struggling to rebuild consumer loyalty after E. coli, Salmonella, and Norovirus outbreaks in 2015 caused a huge damage to its sales and brand.

It has been about two years since an E. coli outbreak rocked the food chain in October and December 2015 in which almost 60 people were infected with the infection after they had eaten at the chain's restaurants across the country. Out of the people who were infected, at least 22 people were hospitalized. This was followed by a norovirus outbreak at a single restaurant in 2015, according to Fortune.

Norovirus is a harmful virus that spreads easily through contaminated food and drink and can have serious implications on people's health. According to the Center for Disease Control and Prevention (CDC), norovirus symptoms typically appear in an infected person within 12-48 hours after he/she is exposed to the virus. The most common symptoms in individuals are diarrhea, nausea, vomiting, and stomach aches.