Strains of the listeria monocytogenes pathogen can survive for up to a year, according to Cornell University scientists who examined strains found in retail stores and processing plants between 1997 and 2002.

The bacterium was found directly on food in 47 of 50 food retailers, and when re-inspected weeks, months and a year later, about 34% had same-strain persistence. Persistent strains were also found in three of seven processing plants. Cleaning does not always eliminate it, but pasteurisation and cooking kill the bacterium found in deli foods like ham and smoked fish.

It was also found in hummus, imitation crab, cheeses, raw turkey and shrimp. Listeria can cause listeriosis, affecting pregnant women, newborns, and adults with weak immune systems, and killing about 500 Americans each year.