Nontuberculous mycobacterial infections such as Mycobacterium avium-complex (MAC) are likely to become disseminated illnesses in immunocompromised persons. In immunocompetent persons, such infections are more likely to resemble tuberculosis. The acid-fast MAC organisms proliferate profusely in macrophages within the mononuclear phagocyte system. Extensive and severe aspergillosis and candidiasis are more likely to occur with profound neutropenia, not lymphocytopenia. Legionella produces an extensive bacterial pneumonia, not disseminated disease. Nocardiosis occurs in immunocompromised patients, and the organisms can be weakly acid-fast, but focal nodules or a chronic abscessing inflammatory response are more likely.