Mexican fast food chain Chipotle announced on Sunday, Nov. 1, that it has shut down all its branches in two markets in the West Coast following a recent E. coli bacteria outbreak.

Chipotle said it had decided to close its restaurants in the cities of Portland and Seattle after investigating some 20 E. coli infection cases reported by health officials involving individuals who ate at six of its branches in these areas.

The food chain also pointed out that there has been no report of such incidents from its other restaurants in the two cities.

The recent outbreak marks the third case of food contamination at Chipotle since August. Previous cases were linked to salmonella and norovirus, a highly contagious disease.

Since opening its first store in 1993, Chipotle has become one of the fastest growing restaurant companies in the United States.

It has set itself apart from other fast food chains by using fresh and healthy ingredients for its selection of tacos, burritos and salads.

Chipotle is part of a growing number of food chains that offer food choices made with fresh and unprocessed ingredients.

Health experts, however, believe the initiative increases the risk of foodborne infections because it eliminates the cooking process in food preparation, which could kill many of the pathogens that cause illnesses.

Three E. coli cases reported in Oregon and 19 in Washington have been linked to consuming food at Chipotle restaurants in October. The ages of infected individuals range from 11 years old to 64 years old.

About a third of customers affected by the outbreak have been admitted to hospitals, but there have been no reports of deaths associated with the incidents.

The Oregon Health Authority, however, said that more reports of E. coli infection cases are likely to be made because some of the customers who may have contracted the intestinal bacteria may not have received medical care yet.

The health agency advises customers, who have eaten at one of Chipotle's stores in the area between Oct. 14 and Oct. 23 and who have become sick with bloody diarrhea and vomiting, to contact their health care provider and report the incident.

What is E. Coli?

Escherichia coli (E. coli) is one of the many bacterial life forms that inhabit the human gut, according to the Centers for Disease Control and Prevention (CDC).

While the organism is generally not known to cause harm and could even be an important part of a healthy intestinal tract, some E. coli strains produce a chemical called Shiga toxin, which can cause people to develop severe symptoms that could lead to death.

What are Symptoms of E. Coli Infections?

People infected by the E. coli bacteria often suffer severe cramping of the stomach, bloody diarrhea and vomiting. Some individuals also experience a slight fever at temperatures that reach less than 101 degrees Fahrenheit (38.3 degrees Celsius).

Most cases of infection can be categorized as mild, with patients making a full recovery within five to seven days. In some cases, however, individuals develop more serious forms of symptoms that can become life-threatening.

How is the Bacteria Transmitted?

E. coli infections begin when an individual ingests the bacteria typically found in human or animal feces, which the CDC says occurs more often than what most people think.

Bacterial exposure that results in sickness occurs when an individual consumes contaminated food and water or raw milk, or has direct contact with infected cattle or E. coli-contaminated human feces.

Health officials caution people from eating food that contains a strain of the bacteria known as E. coli O157. These food items include raw milk, soft cheeses made from unpasteurized milk and unpasteurized apple cider.

Undercooked hamburgers and unprocessed produce could also contain traces of the E. coli bacteria.