Listeria bacteria infecting a HeLa cell. Confocal light micrograph of Listeria sp. bacteria (green) infecting a HeLa cell. The cell's nucleus is blue. Listeria are Gram-positive rod-shaped bacteria. The species L. monocytogenes is a human pathogen, causing the disease listeriosis. Symptoms of listeriosis include fever, meningitis and encephalitis (swelling of the brain). It is caught from contaminated food and is most common in newborns, the elderly and immunocompromised patients. HeLa cells are a continuously cultured human cell line used for biological and medical research. They were cultivated in 1952 from a cancer of the cervix of Henrietta Lacks (after whom the cells were named).