Brucella is a model of an intracellular parasite, a category that includes other important bacteria, such as those of tuberculosis or legionelosis. Brucella penetrates the macrophages within membranous vesicles that are not fused with lysosomes (structures containing cellular products necessary to destroy bacteria) as occurs in other micro-organisms. On the contrary, they reach certain compartments within the macrophage. Here the bacteria multiply and establish a chain of events that determine the illness. Brucellosis, the illness caused by these bacteria, is of great importance worldwide, with millions of human beings and domestic animals affected. This discovery not only means new useful ideas for other researchers, but also the enhanced knowledge of a very important pathogen. From this knowledge useful products, such as new vaccines, can be derived.