 Other bacteria found on lettuce include Aeromonas species , which have not been linked to any outbreaks ; Campylobacter species , which cause campylobacteriosis ; and Yersinia intermedia and Yersinia <unk> ( species of Yersinia ) , which have been found mainly in lettuce . Lettuce has been linked to numerous outbreaks of the bacteria E. coli O157 : H7 and Shigella ; the plants were most likely contaminated through contact with animal feces . A 2007 study determined that the vacuum cooling method , especially prevalent in the California lettuce industry , increased the uptake and survival rates of E. coli O157 : H7 . Salmonella bacteria , including the uncommon Salmonella <unk> type , have also caused outbreaks traced to contaminated lettuce . Viruses , including hepatitis A , <unk> and a Norwalk @-@ like strain , have been found in lettuce . The vegetable has also been linked to outbreaks of parasitic infestations , including Giardia <unk> . 
