Health authorities are working to pinpoint specific products or delis linked to the outbreak.
The contaminated meats may include cold cuts, lunch meats, hot dogs or pates.
Investigators said a tainted meat or cheese probably introduced the outbreak strain into delis in multiple states.
Listeria can easily spread among countertops, deli slicers, surfaces and hands.
It can survive and grow at cold temperatures in the refrigerator.
Illnesses have been recorded in California, Illinois, Maryland, Massachusetts, New Jersey and New York.
Pregnant women and newborns are also at risk of serious infection.
Anyone who experiences symptoms of severe listeria illness should call their health care provider.
Americans 65 and older are four times as likely as others to get a listeria infection.
