Chipotle store front
Chipotle's stock has spiraled downward after the outbreak.

SEATTLE — Chipotle Mexican Grill temporarily has closed 43 restaurants in the Pacific Northwest after more than 20 people became ill with E. coli. The news sent Chipotle’s stock down as much as 5% after the stock market opened on Nov. 2.

“After being notified by health department officials in the Seattle (Wash.) and Portland, Ore., areas that they were investigating approximately 20 cases of E. coli, including people who ate at six of our restaurants in those areas, we immediately closed all of our restaurants in the area out of an abundance of caution,” Chipotle said.

According to the Oregon Health Authority, as of Oct. 31, there were 3 reported cases in Oregon and at least 19 in Washington. One third of the cases have been hospitalized, but there have been no deaths.

The cause of the infection is unknown, but several reports pointed to the fresh, uncooked ingredients that Chipotle uses in its food preparation.

It has been a difficult past few months for Chipotle. In mid-September, state health and agriculture officials investigated an outbreak of salmonellosis associated with eating at Chipotle restaurants in Minnesota. In that case, authorities identified tainted tomatoes as the source.

Chipotle opened with a single restaurant in 1993 and now operates more than 1,900 restaurants, including 22 Chipotle restaurants outside the United States.