Tuberculosis

Tuberculosis is a chronic infectious disease caused by Mycobacterium tuberculosis, transmitted through the air and most often affecting the lungs. It is distinctive for its capacity to persist in a latent, non-contagious state for years before progressing to active, transmissible disease, and it remains one of the world's leading causes of death from a single infectious agent.

Definition

Tuberculosis is infection by Mycobacterium tuberculosis, characterized by granulomatous inflammation that most commonly involves the lungs and that exists along a spectrum from contained latent infection to progressive, contagious active disease.

Scope

This entry covers the causative organism, airborne transmission, the latent-versus-active distinction, granulomatous pathogenesis, the spectrum of pulmonary and extrapulmonary disease, principles of diagnosis, drug resistance, and global epidemiology. It is a reference overview of how tuberculosis is understood and classified, not a diagnostic or treatment protocol.

Key concepts

• Mycobacterium tuberculosis
• Airborne (droplet-nuclei) transmission
• Latent tuberculosis infection
• Active (pulmonary and extrapulmonary) tuberculosis
• Granuloma formation and caseating necrosis
• Cavitation and reactivation
• Drug-resistant tuberculosis (MDR and XDR)

Mechanisms

Mycobacterium tuberculosis is inhaled in droplet nuclei and deposited in the alveoli, where it is taken up by macrophages but resists destruction and replicates intracellularly. A cell-mediated immune response organizes infected macrophages into granulomas that wall off the bacilli; in most people this contains the infection in a latent state without symptoms or transmission. When containment fails — at the time of initial infection or later through reactivation, often with waning immunity — bacilli proliferate, granulomas undergo caseating necrosis, and cavities form in the lung that release organisms into the airways, rendering the person contagious. Hematogenous and lymphatic spread can seed extrapulmonary sites.

Clinical relevance

Tuberculosis is a major focus of pulmonology, infectious disease, and global public health because of its airborne transmission, its latent reservoir, and the challenge of drug resistance. This description explains how the disease is conceptualized and categorized for educational reference and does not constitute guidance for diagnosing or treating an individual.

Epidemiology

Tuberculosis remains among the leading infectious causes of death worldwide, with a large global reservoir of latent infection from which a fraction progress to active disease (Furin, 2019). Risk of progression is amplified by HIV co-infection and other causes of impaired immunity, and the emergence of multidrug-resistant and extensively drug-resistant strains complicates control. The World Health Organization's End TB Strategy set global targets to reduce tuberculosis incidence and deaths (Uplekar, 2015).

History

Tuberculosis, historically known as consumption or phthisis, has afflicted humans for millennia. Robert Koch's 1882 identification of the tubercle bacillus established its infectious cause and is a landmark of microbiology. The sanatorium era preceded effective therapy, and the mid-twentieth-century introduction of streptomycin and subsequent antimycobacterial drugs made cure possible, while the late twentieth century brought the linked challenges of HIV co-infection and drug resistance.

Seminal works

• furin-2019
• lewinsohn-2017

Frequently asked questions

What is the difference between latent and active tuberculosis?

In latent tuberculosis infection the bacteria are contained by the immune system; the person has no symptoms and is not contagious. In active tuberculosis the bacteria multiply and cause illness, and pulmonary active disease can be transmitted to others through the air.

How does tuberculosis spread?

It spreads through the air when a person with active pulmonary tuberculosis coughs, speaks, or sneezes, releasing droplet nuclei containing the bacteria that another person then inhales.