Listeria monocytogenes bacterium, computer illustration. L. monocytogenes is the causative agent of the human disease listeriosis. Listeriosis is contracted through contaminated food. Pregnant women are especially vulnerable, when bacteria may cross the placenta to infect the baby, who has no immunity. A mother may only experience mild influenza-like symptoms yet may lose her baby. Others vulnerable include the elderly, persons with cancer (especially of the bowel) and those on immunosuppressive drugs.