**2. Etiology of human brucellosis**

Numerous names for human brucellosis exist, such as undulant fever, Gibraltar fever, gastric fever, Malta fever, Mediterranean fever, Maltese fever, intermittent fever, and rock fever of Gibraltar. It is a zoonotic disease that has been around for a long time; fresh evidence from ancient Egyptian skeletons suggests it is been around since, at least, 750 BC. The Mediterranean, the southern and central United States, Africa, Asia, the Arab peninsula, the Indian subcontinent, and the Middle East all have a higher prevalence of this infection.

Many different kinds of animals are susceptible to the bacterial zoonotic disease known as brucellosis, which can be passed on to people through infected food or by casual contact. The zoonotic disease brucellosis in humans is mostly ignored. In humans, brucellosis begins as a devastating acute infection that can become chronic and have numerous problems; it is a disease of poverty. Infections in animals have a large socioeconomic impact. The annual number of human cases is commonly reported as 500,000, but this is likely an underestimate due to the fact that many of the worstaffected countries lack the resources to properly diagnose the disease, and because the flu-like symptoms are also present in a number of other febrile illnesses [13].

As a bacterial illness with a global reach, brucellosis affects not just individuals but also the livelihoods of entire communities and economies.

#### *Milk Borne Brucellosis DOI: http://dx.doi.org/10.5772/intechopen.109124*

Several host-specific *Brucella* species are responsible for this disease. In order to spread throughout the body, only need just 10–100 infective organisms. Brucella species are gram-negative cocci bacilli (GNCB) that are nonmotile, do not produce spores, and are encapsulated. They are named after Sir David Bruce (1855–1931), the man responsible for discovering the brucellosis-causing bacteria. Although aerobic, certain bacteria can only be successfully isolated in the presence of 5–10% carbon dioxide. In mammals, *Brucella* organisms concentrate on the reproductive organs, where they cause abortions and infertility. The animal's fluids, such as urine, milk, and placental fluid, are rich in them [13, 14].

So far, twelve distinct *Brucella* species have been identified; while they can spread to a wide variety of hosts, each species has a preferred host. These twelve species consist of six traditional species and six new species. The traditional species are *B. abortus*, *B. melitensis*, *Br. suis*, *Brucella neotomae*, *Brucella ovis*, and *B. canisand*, while the new species are *B. canisand*, *Brucella ceti*, *B.pinnipedialis*, *B.microti*, *B.inopinata*, *B. Papionis*, and *B. vulpis*. Three of the *Brucella* species are known to be endemic in most countries, and they are highly virulent to both their natural hosts and humans. They include *Brucella abortus*, which mostly infects cattle; *B. melitensis*, which primarily infects sheep and goats; and B. suis, which has a tropism for domestic, feral, and wild pig populations [15, 16].
