M. tuberculosis) or fungi. In immunocompromised patients, such as those with debilitating illness, on immunosuppressive regimens, or with human immunodeficiency virus (HIV) infection (see later), the usual presentation is widespread disease due to systemic dissemination of the causative organism. Tuberculosis is by far the most important entity within the spectrum of chronic pneumonias.
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Tuberculosis is a communicable chronic granulomatous disease caused by Mycobacterium tuberculosis. It usually involves the lungs but may affect any organ or tissue in the body.
The World Health Organization (WHO) considers tuberculosis to be the most common cause of death resulting from a single infectious agent. It is estimated that 1.7 billion individuals are infected worldwide, with 8 to 10 million new cases and 1.5 million deaths per year. In the Western world, deaths from tuberculosis peaked in 1800 and steadily declined throughout the 1800s and 1900s. However, in 1984 the decline in new cases stopped abruptly, a change that resulted from the increased incidence of tuberculosis in HIV-infected individuals. As a consequence of intensive public health surveillance and tuberculosis prophylaxis among immunosuppressed individuals, the incidence of tuberculosis in U.S.-born individuals has declined since 1992. In 2014, there were approximately 9400 new cases of active tuberculosis in the United States, with nearly 60% of these occurring in immigrants from countries where tuberculosis is endemic.
Tuberculosis flourishes under conditions of poverty, crowding, and chronic debilitating illness. In the United States, tuberculosis is a disease of older adults, the urban poor, patients with AIDS, and members of minority communities. African Americans, Native Americans, the Inuit (from Alaska), Hispanics, and immigrants from Southeast Asia have higher rates of disease than other segments of the population. Certain disease states also increase the risk, such as diabetes mellitus, Hodgkin lymphoma, chronic lung disease (particularly silicosis), chronic renal failure, malnutrition, alcoholism, and immunosuppression. In areas of the world where HIV infection is prevalent, HIV infection is the dominant risk factor for the development of tuberculosis.
It is important that infection be differentiated from disease. Infection implies seeding of a focus with organisms, which may or may not cause clinically significant tissue damage (i.e., disease). Although other routes may be involved, most infections are acquired by direct person-toperson transmission of airborne droplets of organisms from an active case to a susceptible host. In most individuals, an asymptomatic focus of pulmonary infection appears that is self-limited, although uncommonly, primary tuberculosis may result in the development of fever and pleural effusions. Generally, the only evidence of infection, if any remains, is a tiny, telltale fibrocalcific nodule at the site of the infection. Viable organisms may remain dormant in such loci for decades, and possibly for the life of the host. Such individuals are infected but do not have active disease and therefore cannot transmit organisms to others. Yet, if their immune defenses are lowered, the infection may reactivate to produce communicable and potentially life-threatening disease.