Only 5 to 10% of immunocompetent humans are susceptible to tuberculosis, and over 85% of them develop the disease exclusively in the lungs. Human immunodeficiency virus (HIV)-infected humans, in contrast, can develop systemic disease that is more quickly lethal. This is in keeping with other evidence showing that susceptible humans generate some level of Th1 immunity to Mycobacterium tuberculosis (Mtb) infection. Tuberculosis in mice is also exclusively a lung disease that is progressive and lethal, in spite of the generation of Th1-mediated immunity. Thus mouse tuberculosis is a model of tuberculosis in susceptible humans, as is tuberculosis in guinea pigs and rabbits. Inability to resolve infection and prevent disease may not be a consequence of the generation of an inadequate number of Th1 cells but of an intrinsic deficiency in macrophage function that prevents these cells from expressing immunity. If this proves to be true, vaccinating susceptible humans against tuberculosis will be a difficult task.