Listeriosis usually results from infection by Listeria monocytogenes, a Gram positive rod in the family Listeriaceae. This organism is a facultative intracellular pathogen. There are 13 serovars of L. monocytogenes. Although all are considered to be potentially virulent, serovars 4b, 1/2b, and 1/2a cause most animal and human disease. L. ivanovii (formerly known as L. bulgarica or serovar 5 of L. monocytogenes) is occasionally associated with abortions in sheep and cows, or septicemia in sheep. Rare infections with L. ivanovii and L. seeligeri have been reported in humans. L. innocua, L. welshimeri and L. grayi have not been associated with human disease.