Chipotle Virus Outbreak Hits 120, School Says: What You Need to Know About Norovirus in Restaurants

— -- The norovirus outbreak at a Chipotle that left at least 120 Boston College students sick, according to school officials, has put a spotlight on the contagious virus that experts say often spreads throughout restaurants.

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.

An estimated 70 percent of norovirus outbreaks are caused by infected workers who contaminated 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 Hospital Case Medical Center in Cleveland, explained 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. "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 it is 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; 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 avoiding touching ready-to-eat foods. They also recommend requiring sick food workers to stay home.

Chipotle has announced multiple new safety measures in recent weeks after a multi-state E.coli outbreak_" target="_blank">E. coli outbreak infected at least 52, according to the CDC.

The outbreak was likely norovirus, the company acknowledged in a statement to ABC News Tuesday.

"It is important to note that noroviruses are very common, in part because they are so easily transmitted (they can spread through person-to-person contact, on surface areas, or through food or drink)," a Chipotle spokesman said in the statement.

The company announced last month it has started new 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.

There are about 20 million cases a year of norovirus, making it the leading cause of gastroenteritis in the United States, according to the CDC.