Tuberculosis in depth.

Overview

Tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium tuberculosis — a slow-growing, acid-fast bacillus that has co-evolved with humans for thousands of years. TB primarily affects the lungs (pulmonary TB), accounting for the majority of cases, but can affect virtually any organ in the body including the lymph nodes, brain (TB meningitis), spine (Pott's disease), kidneys, bones, and skin (extrapulmonary TB). Pulmonary TB is the infectious form; extrapulmonary TB is generally not contagious.

TB remains one of humanity's greatest infectious disease burdens. The WHO estimated 10.6 million people developed TB in 2022, and 1.3 million died from the disease — making it the world's deadliest bacterial infectious disease. An extraordinary feature of TB epidemiology is the scale of latent infection: approximately one-quarter of the global population (around 1.7 billion people) carry M. tuberculosis in a latent, non-infectious state. Of these, approximately 5–10% will develop active TB disease during their lifetime — a risk dramatically elevated (up to 10% per year) in people living with HIV.

Drug resistance is the defining challenge of modern TB control. Multi-drug resistant TB (MDR-TB), defined as resistance to at least isoniazid and rifampicin, affected an estimated 410,000 people in 2022. Extensively drug-resistant TB (XDR-TB), resistant to additional second-line drugs, represents one of the most difficult-to-treat infectious diseases in clinical medicine. The WHO End TB Strategy aims to reduce TB deaths by 95% and new cases by 90% between 2015 and 2030 — targets that require massive acceleration of current progress.

History & Origin

Tuberculosis is one of the oldest documented human diseases. M. tuberculosis DNA has been identified in Egyptian mummies dating to 3000 BCE, and ancient Greek physicians described the disease they called "phthisis" (consumption). Hippocrates noted it as the most prevalent disease of his era. The historical name "consumption" described the way the disease appeared to consume its victims from within — the dramatic weight loss, pallor, and slow decline.

Robert Koch identified Mycobacterium tuberculosis as the causative agent in 1882 — one of the landmark achievements in the history of medicine — earning him the Nobel Prize in Physiology or Medicine in 1905. The development of streptomycin by Selman Waksman in 1943 provided the first effective antibiotic treatment. The BCG vaccine, developed by Albert Calmette and Camille Guérin and first used in humans in 1921, remains the only TB vaccine in widespread use more than a century later. The modern era of TB chemotherapy was established in the 1950s–1960s with isoniazid, pyrazinamide, ethambutol, and rifampicin — the four drugs that remain the backbone of standard TB treatment today.

The HIV epidemic beginning in the 1980s dramatically worsened TB globally, as HIV destroys the CD4+ T cells critical for immune control of latent TB. In high-HIV-burden countries like South Africa, TB became the leading cause of death in people living with HIV. Drug resistance, amplified by incomplete treatment courses, emerged as a major threat during the 1990s.

Transmission

TB is spread through the air via inhalation of infectious droplet nuclei containing M. tuberculosis. These tiny particles (1–5 micrometers in diameter) are produced when a person with active pulmonary or laryngeal TB coughs, sneezes, speaks, or sings. They can remain suspended in the air for hours in poorly ventilated spaces.

• Primary risk factor: Prolonged close contact with an infectious pulmonary TB case — household members, coworkers, or others sharing enclosed spaces with poor ventilation.
• Infectiousness: Smear-positive pulmonary TB cases (visible bacteria under microscope) are most infectious. A single cough can produce 3,000 infectious droplet nuclei. Untreated, one person with active TB can infect 10–15 others per year on average.
• TB is NOT spread by: Touching surfaces, shaking hands, sharing food or drink (brief casual contact is very low risk).
• Factors increasing transmission risk: Small, poorly ventilated spaces; high density living or working conditions; HIV co-infection (prolongs period of infectiousness); delayed diagnosis.
• Factors reducing transmission: Effective TB treatment (infectious period decreases dramatically within 2–4 weeks of starting treatment); natural ventilation; UV germicidal irradiation; N95 respirators in healthcare settings.

Symptom Timeline

TB has a complex natural history shaped by the balance between bacterial virulence and host immune response. After initial infection, most people develop latent TB with no symptoms.

Diagnosis

• Sputum smear microscopy: Detection of acid-fast bacilli (AFB) in sputum — rapid, inexpensive, widely available. Sensitivity approximately 45–80% for pulmonary TB; misses paucibacillary disease (HIV co-infected patients, children).
• GeneXpert MTB/RIF (Xpert): WHO-recommended rapid molecular test that detects M. tuberculosis DNA and rifampicin resistance simultaneously within 2 hours. Sensitivity ~89% for smear-positive, ~67% for smear-negative TB. Now the preferred first-line test in high-burden settings.
• Culture: Gold standard for TB diagnosis and drug susceptibility testing. MGIT liquid culture takes 10–21 days. Required for complete drug susceptibility testing (DST) in suspected MDR-TB.
• Tuberculin Skin Test (TST/Mantoux): Injection of purified protein derivative (PPD); induration ≥10mm after 48–72 hours indicates sensitization to TB antigens. Does not distinguish active from latent TB; false positives in BCG-vaccinated persons.
• Interferon-Gamma Release Assays (IGRAs — QuantiFERON, T-SPOT.TB): Blood tests detecting interferon-gamma release by T cells in response to TB-specific antigens. More specific than TST; not affected by BCG vaccination. Used for LTBI diagnosis.
• Chest X-ray: Upper lobe cavitation, nodular infiltrates, and hilar lymphadenopathy are classic findings. Normal X-ray does not exclude TB. CT scan provides greater detail for complex cases.
• Whole genome sequencing (WGS): Increasingly used for comprehensive drug resistance profiling and outbreak investigation.

Treatment

TB is treatable and curable. Standard treatment follows the DOTS (Directly Observed Treatment, Short-course) strategy endorsed by WHO. Treatment success rates exceed 85% for new drug-susceptible TB cases globally.

Drug-Susceptible TB (6 months)

• Intensive phase (2 months): Isoniazid (H) + Rifampicin (R) + Pyrazinamide (Z) + Ethambutol (E) — HRZE
• Continuation phase (4 months): Isoniazid (H) + Rifampicin (R) — HR
• Treatment completion is essential — stopping early causes relapse and drug resistance development
• Fixed-dose combination tablets improve adherence and reduce medication errors

MDR-TB Treatment

• Requires 9–20 months of second-line drugs including bedaquiline, linezolid, and fluoroquinolones
• WHO-recommended shorter regimens (BPaLM — bedaquiline, pretomanid, linezolid, moxifloxacin) show 89% success in 6 months for eligible MDR-TB patients
• Treatment success rates for MDR-TB: approximately 57% globally (WHO 2022)
• XDR-TB (resistant to fluoroquinolones and other second-line drugs): BPaL regimen (bedaquiline, pretomanid, linezolid) — 89% treatment success in ZeNix/ZeStrial trials

Latent TB Treatment (LTBI)

• 3 months of isoniazid + rifampicin (3HR) or 1 month of isoniazid + rifapentine (1HP) — now preferred short regimens
• 6 months of isoniazid (6H) — long-established regimen with extensive evidence base
• Recommended for HIV-positive persons, household contacts of active TB, and other high-risk groups

Prevention & Vaccines

• BCG vaccine: Bacille Calmette-Guerin — the only TB vaccine approved for use. Provides ~80% protection against severe forms of TB in children (TB meningitis, miliary TB). Variable protection against adult pulmonary TB. Recommended for newborns in high-TB-burden countries by WHO.
• Treatment of latent TB infection (LTBI): Treating LTBI prevents progression to active TB — the most effective tool for reducing TB incidence in high-risk individuals.
• Infection control: Natural ventilation, UV germicidal irradiation, and N95 respirators in healthcare settings reduce transmission. Prompt diagnosis and treatment rapidly reduces infectiousness.
• New vaccine candidates: M72/AS01E (GlaxoSmithKline/PATH) showed 49.7% efficacy against active TB in adults with LTBI in Phase 2b trial — a promising candidate in Phase 3 trials. BCG revaccination and several viral-vectored candidates also in trials.
• Social determinants: Adequate nutrition, housing, income, and reduction of HIV co-infection are critical for TB elimination — TB is strongly associated with poverty and crowding.

Global Impact

Tuberculosis is the world's deadliest bacterial infectious disease and ranked among the top infectious disease killers globally alongside HIV/AIDS. The WHO estimated 10.6 million new TB cases in 2022, of which 5.8 million were in men, 3.5 million in women, and 1.3 million in children. An estimated 1.3 million people died from TB in 2022 (including 167,000 people with HIV).

Eight countries account for two-thirds of all TB cases: India (27%), Indonesia (10%), China (7.1%), Philippines (7.0%), Pakistan (5.7%), Nigeria (4.5%), Bangladesh (3.6%), and Democratic Republic of Congo (2.9%). South Africa, with a severe HIV co-epidemic, has exceptionally high TB mortality rates despite not being in the top case-count countries.

Drug resistance represents the most critical threat to TB control. In 2022, approximately 410,000 people developed MDR/RR-TB, but only 176,000 were enrolled on appropriate treatment. The global MDR-TB treatment success rate of 57% leaves enormous room for improvement. Pre-XDR-TB and XDR-TB require expensive, toxic, prolonged treatment regimens and carry treatment success rates below 50% in many settings.

The COVID-19 pandemic severely disrupted TB services globally — TB case notifications fell by approximately 18% in 2020, and an estimated 1.4 million additional TB deaths occurred as a direct consequence of service disruptions. Recovery of TB services to pre-pandemic levels has been slow in many high-burden countries.

Country-Specific Information

India: India has the world's highest TB burden — an estimated 2.8 million new cases in 2022 (27% of global total) and over 300,000 deaths. India launched the National TB Elimination Programme with an ambitious target of eliminating TB by 2025 (ahead of the global 2030 SDG target). Nikshay Poshan Yojana provides nutritional support to TB patients.

South Africa: South Africa has one of the world's highest TB notification rates per capita, driven by the HIV epidemic — approximately 60% of South African TB patients are HIV-positive. MDR-TB is highly prevalent. The country is a priority focus for the WHO/Global Fund TB response.

China: China has reduced TB incidence significantly over three decades through sustained DOTS implementation. However, MDR-TB remains a serious concern, particularly in provinces bordering high-burden countries. China now accounts for approximately 7% of global TB cases.

Indonesia: Indonesia is the second-highest TB burden country globally. Significant challenges include limited laboratory capacity, high proportion of undiagnosed cases, and growing MDR-TB burden. The country has expanded GeneXpert deployment to improve rapid diagnosis.

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