Chipotle Facing Investigation Over Viral Outbreak: What You Should Know About Norovirus

— -- The Chipotle Mexican Grill restaurant chain is facing scrutiny by federal authorities after a viral outbreak in Simi Valley, California, last year led to people being sickened by the company's food, a situation that is bringing attention to the extremely contagious norovirus.

A norovirus outbreak in Simi Valley connected to a Chipotle restaurant is being investigated by U.S. Attorney’s Office for the Central District of California, in conjunction with the U.S. Food and Drug Administration’s Office of Criminal Investigations, according to a U.S. Securities and Exchange Commission filing today. Last year, the chain dealt with multiple outbreaks, including a widely covered E.coli outbreak that sickened at least 53 people in multiple states.

A company spokesman declined to respond to the legal action.

"As a matter of policy, we don't discuss pending legal action, but we will cooperate fully with this investigation," Chipotle spokesman Chris Arnold told ABC News.

The news about the criminal probe has drawn attention to norovirus and how it spreads. Norovirus is the number one cause of outbreaks from contaminated food in the United States and affects an estimated 20 million Americans every year, according to the Centers for Disease Control and Prevention, making it the leading cause of gastroenteritis in the United States.

An estimated 70 percent of norovirus outbreaks are caused by infected workers who contaminate the food. Experts explain that the virus is contagious and can remain on an uncleaned surface for weeks.

Dr. Amy Edwards, an infectious disease specialist at University Hospitals Case Medical Center in Cleveland, told ABC News in an earlier interview that unlike other viruses, norovirus can spread if a person comes into contact with just a few virus particles.

"You only need a handful; two, three, four [virus particles]," Edwards told ABC News in December. "If [an infected person] throws up they can spread a million or more virus particles."

Edwards said the reason restaurants and cruise ships are classically affected by the virus is because they are places where people are grouped together and can be exposed. Edwards pointed out that if a restaurant customer is sick and touches a table, another person can be infected if he or she touches the contaminated table and then eats food, like a burrito, by hand.

"A set up where you’re eating a burrito with your hands is a classic set up; [it] can spread the norovirus," Edwards said. "You really have to use chemicals" to clean the tables of the virus.

The CDC recommends that restaurants guard against outbreaks by having food service workers practice proper hand washing and avoid touching ready-to-eat foods. They also recommend requiring sick food workers to stay home.

After multiple outbreaks last year, Chipotle has announced new safety measures to ensure food safety and handling, including testing fresh produce with DNA-based tests, and end-of-shelf-life testing to ensure ingredients are safe throughout their shelf life, while also looking to improve the supply chain by measuring performance data of vendors and suppliers, and enhancing employee training in food safety and handling.