Listeria is a genus of bacteria that acts as an intracellular parasite in mammals. Until 1992, 17 species were known, each containing two subspecies. By 2020, 21 species had been identified. The genus is named in honour of the British pioneer of sterile surgery Joseph Lister. Listeria species are Gram-positive, rod-shaped, and facultatively anaerobic, and do not produce endospores. The major human pathogen in the genus Listeria is L. monocytogenes. It is usually the causative agent of the relatively rare bacterial disease listeriosis, an infection caused by eating food contaminated with the bacteria. Listeriosis can cause serious illness in pregnant women, newborns, adults with weakened immune systems and the elderly, and may cause gastroenteritis in others who have been severely infected. Listeriosis is a serious disease for humans; the overt form of the disease has a case-fatality rate of around 20%. The two main clinical manifestations are sepsis and meningitis. Meningitis is often complicated by encephalitis, when it is known as meningoencephalitis, a pathology that is unusual for bacterial infections. L. ivanovii is a pathogen of mammals, specifically ruminants, and has rarely caused listeriosis in humans. The incubation period can vary from three to 70 days. Background The first documented case of listeriosis was in 1924. In the late 1920s, two researchers independently identified L. monocytogenes from animal outbreaks. They proposed the genus Listerella in honour of surgeon and early antiseptic advocate Joseph Lister, but that name was already in use for a slime mould and a protozoan. Eventually, the genus Listeria was proposed and accepted. All species within the genus Listeria are Gram-positive, catalase-positive rods and do not produce endospores. The genus Listeria was classified in the family Corynebacteriaceae through the seventh edition (1957) of Bergey's Manual of Systematic Bacteriology. The 16S rRNA cataloging studies of Stackebrandt, et al. demonstrated that L. monocytogenes is a distinct taxon within the Lactobacillus-Bacillus branch of the bacterial phylogeny constructed by Woese. In 2004, the genus was placed in the newly created family Listeriaceae. The only other genus in the family is Brochothrix. The genus Listeria is known to contain 21 species: L. aquatica, L. booriae, L. cornellensis, L. costaricensis, L. goaensis, L. fleischmannii, L. floridensis, L. grandensis, L. grayi, L. innocua, L. ivanovii, L. marthii, L. monocytogenes, L. newyorkensis, L. riparia, L. rocourtiae, L. seeligeri, L. thailandensis, L. valentina, L. weihenstephanensis, and L. welshimeri. Listeria dinitrificans, previously thought to be part of