Brucellae are tiny, aerobic, slow growing, catalase and oxidase positive Gram negative coccobacilli or small rods, which may reach man through exposure to tissues of mammalian hosts via cuts or aerosols, or as food infections mostly through dairy products. As parasites brucellae are extraordinarily successful, causing very long-lasting infections in all mammalian social animals, such as ungulates, canids, and rodents; recently they have been found to also cause disease in pinnipeds and cetaceans. Brucellae as members of the alpha Proteobacteria, have suffered major losses of genomic material as they adapted to their facultative intracellular parasite role, and are able to initiate infection with minimal disturbance of the innate immune system, thus reaching a privileged intracellular niche where they multiply. Brucellae are likely to be among the toughest organisms to control through public health and agricultural policies, even involving detection-slaughter strategies. Copyright © Informa Healthcare USA, Inc.