"These data suggest that failure to thoroughly execute cleaning and sanitation protocols is allowing L. monocytogenes to persist in some stores," she added.

Listeriosis -- the illness caused by L. monocytogenes bacteria -- causes fever and chills, headache, upset stomach and vomiting, according to the U.S.

The outbreak began when seven cases of listeriosis—food poisoning caused by L. monocytogenes—hit Colorado hospitals in late August 2011.