At least one school district has already warned parents that food served in its cafeterias was recalled because of possible E. coli contamination.

WSJ: Recalled frozen food may have ended up in schools

Rich Products two weeks ago announced a voluntary recall of certain Farm Rich and Market Day brand products because of the possible E. coli contamination.

WSJ: Recalled frozen food may have ended up in schools

On Tuesday, Hudson recalled 20, 000 pounds -- 80, 000 frozen hamburger patties -- after 16 Colorado consumers became sick from grilled burgers linked to E. coli contamination.

CNN: Recall of frozen hamburger patties doubles

Contamination on farms and in processing facilities from bacteria such as salmonella, E. coli and listeria are often behind the large food recalls, but often food contamination takes place in home and restaurant kitchens, said Dr. Griffin.

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