How did Listeria get into recalled veggies?

Once this bacteria gets into a food processing facility, it can live for years

From: http://redirect.viglink.com?u=http%3A%2F%2Fwww.cbsnews.com%2Fnews%2Fcountry-fresh-recall-listeria-in-vegetables%2F&key=ddaed8f51db7bb1330a6f6de768a69b8