An infectious, zoonotic disease of various animals and humans caused by Brucella species. Each species tends to preferentially infect a particular animal, but several types can infect humans. Brucella melitensis (preferentially infects goats and sheep), B. suis (infects pigs), and B. abortus (infects cattle) are the most common causes of human brucellosis. Brucella melitensis is the most virulent for humans, followed by B. suis and B. abortus. Brucella canis and B. ovis, which infect dogs and sheep respectively, rarely infect humans. Although brucellosis is found all over the world, in many countries the disease has been eradicated. The brucellae are small, gram-negative coccobacilli which are defined as facultative intracellular parasites since they are able to replicate within specialized cells of the host.