The properties of Brucella species for causing an infectious disease show some variability which depends on host immune responses. For example, although, B.canis causes abortus in dogs, it has no obvious effects at human to cause an infectious disease. Conversely, B.melitensis causes an infection in human which may causes death and not in the dogs to make an infection. Therefore, it is suggested that, the reason of this difference between the Brucella species is the differences between the host immune responses. Understanding the interaction between the organism and host immune response is not only participates solving the problem that encountered in the treatment of brucellosis but also gives clues to find a suitable strain for developing an effective vaccine. The investigations in this field is limited with the studies on immunocompetent hosts or experiments and not allowed studies with immunocompromised hosts. In this review, Brucella infection and impaired immunity is discussed and the data in the literature presents together with our experiences to the investigators working in this area.