Listeriosis is a rare but serious disease caused by the bacteria Listeria monocytogenes which is found in soil, water, and on certain high-risk foods. It is a foodborne illness that can be fatal for unborn babies in pregnant women, people older than 65, and people with weakened immune systems. It is most commonly caused by eating improperly processed deli meats and unpasteurized milk products. ELISA PCR and bacteria culture are commonly used for detecting Listeria in food and water samples.