Listeria is a genus of bacteria that contains eight species. Named after the English pioneer of sterile surgery Joseph Lister, the genus received its current name in 1940. Listeria species are Gram-positive bacilli. The major human pathogen in the Listeria genus is L. monocytogenes. It is usually the causative agent of the relatively rare bacterial disease, listeriosis, a serious infection caused by eating food contaminated with the bacteria.