Tuberculosis

Tuberculosis (TB) is an ancient, slow-burning infection caused by Mycobacterium tuberculosis — a bacterium so successful that it quietly infects roughly a quarter of all people alive today. Most carry it silently for a lifetime; a minority go on to develop the wasting, coughing, lung-destroying illness that killed more people over the last two centuries than any other single pathogen. Understanding TB means understanding a peculiar organism that hides inside our own immune cells, grows agonizingly slowly, and demands months of multi-drug therapy to eradicate.

TB matters enormously for the modern clinician and student. It remains a leading infectious killer worldwide, it is curable when managed correctly, and every mishandled case risks breeding drug resistance that can render the disease nearly untreatable. Learning TB well trains a core clinical skill: reasoning about a chronic infection where the diagnosis is often uncertain, the treatment is long, and adherence is everything.

Learning Objectives

• Explain how Mycobacterium tuberculosis is transmitted and how its biology shapes disease.
• Distinguish latent TB infection from active TB disease and why the difference drives management.
• Recognize the clinical features of pulmonary and common extrapulmonary TB.
• Select and interpret diagnostic tests: sputum smear, culture, GeneXpert, IGRA, and the tuberculin skin test.
• Describe standard RIPE therapy, its key toxicities, and the principles of treating resistant disease.
• Understand the special relationship between TB and HIV.

Quick Answer

Tuberculosis is a chronic infection caused by Mycobacterium tuberculosis, spread person-to-person through airborne droplet nuclei from someone with active pulmonary disease. After inhalation, the immune system usually walls the organism off, producing latent TB infection — the person is not sick and not contagious, but harbors dormant bacteria. In about 5–10 percent of immunocompetent people (and far more in those with HIV), the infection reactivates or progresses to active TB, most often in the lungs, causing chronic cough, fever, night sweats, and weight loss. Diagnosis relies on sputum microscopy, mycobacterial culture, and molecular tests such as GeneXpert MTB/RIF that also detect rifampicin resistance. Standard treatment for drug-susceptible disease is six months of combination therapy — two months of rifampicin, isoniazid, pyrazinamide, and ethambutol (RIPE), then four months of rifampicin and isoniazid. Drug-resistant TB (MDR/XDR) requires longer, more toxic regimens and is a major global threat.

Where It Came From

TB has haunted humanity for millennia — its DNA has been recovered from Egyptian mummies and Neolithic bones. In the eighteenth and nineteenth centuries it became the great killer of crowded, industrializing cities, called "consumption" or "phthisis" for the way it consumed its victims, and "the white plague" for the pallor it caused. It was so pervasive that it shaped Romantic art and literature; roughly one in four deaths in nineteenth-century Europe was due to TB.

The scientific turning point came on 24 March 1882, when Robert Koch announced to the Berlin Physiological Society that he had identified the tubercle bacillus as the cause — a landmark that founded modern medical microbiology and is still commemorated as World TB Day. For decades treatment meant little more than fresh air and rest in sanatoria. The real revolution was pharmacological: streptomycin (1944), then isoniazid and pyrazinamide, then rifampicin in the 1960s, turned a death sentence into a curable disease. The early years also taught a hard lesson still central today — treating with a single drug rapidly selected for resistant mutants, which is why TB is always treated with multiple drugs at once. The 1980s HIV pandemic reignited TB globally and drove the emergence of multidrug-resistant strains, making the disease once again one of the world's deadliest infections.

The Organism and How Disease Develops

Mycobacterium tuberculosis is a slow-growing, aerobic bacillus with a thick, waxy, lipid-rich cell wall (rich in mycolic acids). That wall makes it resist standard Gram staining — instead it is "acid-fast," retaining dye even after acid-alcohol washing (the Ziehl-Neelsen stain), appearing as red rods. The same waxy coat helps it survive inside macrophages and explains both its slow growth (dividing every 15–20 hours versus minutes for many bacteria) and the need for prolonged therapy.

Transmission is airborne. A person with active pulmonary TB coughs out tiny droplet nuclei that can hang in the air; another person inhales them into the alveoli. Alveolar macrophages engulf the bacilli but often cannot kill them. Over 2–8 weeks, cell-mediated immunity (T lymphocytes and macrophages) organizes a granuloma — a walled-off ball of immune cells with central caseous ("cheese-like") necrosis. This contains the infection and marks latent TB infection (LTBI): bacteria survive in a dormant state, the person is asymptomatic and non-infectious, and tests of immune sensitization (skin test or IGRA) turn positive.

The Ghon focus (the initial lung lesion) plus involved hilar lymph nodes form the Ghon complex, the classic primary infection footprint. If immunity later wanes — from HIV, aging, diabetes, malnutrition, steroids, or TNF-inhibitors — the granuloma breaks down and bacteria multiply: reactivation (post-primary) TB. This typically strikes the oxygen-rich lung apices, where cavities form and spill huge numbers of bacilli into the airways, making the patient highly infectious.

Worked example: latent versus active

A 30-year-old nurse has a positive IGRA on routine screening, feels completely well, and has a normal chest X-ray. This is latent TB — treat to prevent future disease (e.g., isoniazid for several months or a short rifamycin-based regimen), not with full RIPE. Contrast with a 45-year-old man with three weeks of productive cough, drenching night sweats, 6 kg weight loss, and an upper-lobe cavity on X-ray with acid-fast bacilli in sputum: this is active pulmonary TB — isolate and start four-drug therapy.

Clinical Syndromes: Pulmonary and Beyond

Pulmonary TB is the commonest and most transmissible form. The hallmark is a persistent cough lasting more than 2–3 weeks, often with sputum, sometimes with hemoptysis (coughing blood from eroded vessels in a cavity). Systemic "constitutional" features are typical and important: low-grade fever, drenching night sweats, anorexia, and progressive weight loss (hence "consumption"). Anyone with a cough lasting more than two to three weeks, especially with these features, should be evaluated for TB.

Extrapulmonary TB accounts for roughly 15–20 percent of cases in immunocompetent people and much more in HIV. Key forms:

• Lymph node TB (scrofula): painless, firm, sometimes matted cervical nodes that may discharge. The most common extrapulmonary site.
• Pleural TB: an exudative, lymphocyte-predominant pleural effusion; elevated adenosine deaminase (ADA) supports the diagnosis.
• TB meningitis: subacute headache, fever, meningism, cranial nerve palsies; CSF shows high protein, low glucose, and lymphocytosis. A neurological emergency with high mortality — treat empirically if suspected.
• Miliary TB: widespread bloodstream dissemination producing countless tiny "millet seed" lesions on chest imaging; a severe, multi-organ illness.
• Spinal TB (Pott disease): vertebral destruction that can cause gibbus deformity and cord compression.
• Genitourinary, adrenal, gastrointestinal, and pericardial TB also occur.

Diagnosis: Distinguishing Infection from Disease

The single most useful principle: latent infection and active disease need different tests.

For latent TB, we test immune sensitization:

• Tuberculin skin test (TST/Mantoux): intradermal PPD; read induration (not redness) at 48–72 hours. Cutoffs vary by risk group. False positives occur after BCG vaccination or with non-tuberculous mycobacteria.
• Interferon-gamma release assays (IGRA): blood tests (e.g., QuantiFERON) measuring T-cell interferon-gamma response to TB-specific antigens; not affected by prior BCG.

Neither test distinguishes latent from active disease — a positive result plus symptoms or an abnormal X-ray must trigger a search for active TB.

For active TB, we look for the organism itself:

• Sputum smear microscopy (Ziehl-Neelsen or fluorescent): fast and cheap, but needs a high bacterial load and cannot tell TB from other mycobacteria.
• Nucleic acid amplification / GeneXpert MTB/RIF: rapid (hours) detection of TB DNA and simultaneous rifampicin resistance; now a frontline test.
• Mycobacterial culture: the gold standard and most sensitive, and it enables full drug-susceptibility testing — but slow, taking 2–6 weeks because the organism grows so slowly.
• Chest X-ray / CT: upper-lobe infiltrates, cavities, or a miliary pattern support the diagnosis but are not specific.
• Histology of a biopsy showing caseating granulomas supports extrapulmonary disease.

Treatment and the Problem of Resistance

Drug-susceptible active TB is treated with the RIPE regimen:

Phase	Drugs	Duration
Intensive	Rifampicin, Isoniazid, Pyrazinamide, Ethambutol	2 months
Continuation	Rifampicin, Isoniazid	4 months

Two principles are non-negotiable. First, use multiple drugs together — because spontaneous resistant mutants exist for any single drug, combination therapy prevents them from taking over. Second, complete the full course — stopping early leaves surviving bacteria that regrow, often resistant. Directly observed therapy (DOT) and now digital adherence tools exist precisely because six months of pills is hard to finish.

Key toxicities to memorize:

• Isoniazid: peripheral neuropathy (give pyridoxine/vitamin B6 to prevent it) and hepatitis.
• Rifampicin: harmless orange-red discoloration of urine, tears, and sweat (warn the patient), hepatotoxicity, and powerful induction of liver enzymes causing many drug interactions (notably reducing oral contraceptive and antiretroviral levels).
• Pyrazinamide: hepatotoxicity and hyperuricemia (can trigger gout).
• Ethambutol: optic neuritis — dose-dependent loss of color vision and acuity; monitor vision.

Drug-resistant TB is the great modern threat. Multidrug-resistant TB (MDR-TB) is resistant to at least rifampicin and isoniazid, the two most important drugs. Extensively drug-resistant TB (XDR-TB) adds resistance to fluoroquinolones and other key second-line agents. These require longer regimens of more toxic, less effective drugs; newer agents such as bedaquiline and pretomanid have improved shorter, all-oral MDR-TB regimens. Resistance is largely man-made — the product of incomplete or erratic treatment — which is why stewardship and adherence support are lifesaving public-health tools, not just individual conveniences.

TB and HIV

HIV is the strongest known risk factor for progression from latent to active TB, raising the lifetime risk from about 5–10 percent to roughly that per year. TB is a leading cause of death in people with HIV. Two practical points dominate management: co-infected patients should receive both antiretroviral therapy and TB treatment (with attention to overlapping toxicities and rifampicin's drug interactions), and starting antiretrovirals can provoke immune reconstitution inflammatory syndrome (IRIS), a paradoxical worsening as the recovering immune system attacks the organisms. Screening every TB patient for HIV, and every HIV patient for TB, is standard practice.

Real-World Applications

• Public health: contact tracing around an infectious index case, and treating latent infection in close contacts to break transmission chains.
• Occupational health: IGRA/TST screening of healthcare workers and airborne-isolation (negative-pressure rooms, N95 respirators) for inpatients with suspected pulmonary TB.
• Travel and migration medicine: screening for latent TB in people from high-burden regions before immunosuppressive therapy.
• Chronic disease clinics: heightened vigilance in patients with diabetes, on TNF-inhibitors, or on long-term steroids, all of which reactivate latent disease.
• Global programs: DOT and molecular diagnostics deployed to cure disease and slow resistance.

Common Mistakes

• Treating a positive skin test or IGRA as proof of active disease. These tests only show the immune system has met TB antigens; they cannot distinguish latent infection from active disease, and they do not indicate contagiousness. The correction: interpret them with symptoms, imaging, and microbiology, and give latent-TB treatment (not full RIPE) when disease is excluded.
• Treating TB with a single drug or stopping when the patient feels better. Monotherapy rapidly selects for resistant mutants, and symptoms improve long before the bacteria are eradicated. The correction: always use combination therapy for the full prescribed course, using adherence support.
• Forgetting drug-specific toxicities. Missing that isoniazid causes neuropathy (preventable with B6) or that ethambutol causes optic neuritis can cause avoidable harm. The correction: give pyridoxine with isoniazid, monitor vision on ethambutol, and check liver function on the hepatotoxic drugs.
• Assuming a negative sputum smear rules out TB. Smear needs a high bacterial load and is often negative in early, paucibacillary, or extrapulmonary disease. The correction: use culture and molecular tests, which are far more sensitive.

Comparison and Connections

Feature	Latent TB Infection	Active TB Disease
Symptoms	None	Cough, fever, night sweats, weight loss
Contagious	No	Yes (pulmonary forms)
Chest X-ray	Usually normal	Often abnormal (infiltrate, cavity)
Sputum smear/culture	Negative	Often positive
TST/IGRA	Positive	Positive
Treatment	Single-drug preventive regimen	Multi-drug RIPE therapy

TB is often confused with other cavitary or granulomatous lung diseases such as lung cancer, fungal infection, and sarcoidosis; microbiological confirmation resolves the differential. It also contrasts with non-tuberculous mycobacteria (e.g., M. avium complex), which are acid-fast but environmentally acquired and managed differently. For pathophysiology background see granuloma formation in ../../4._Pathology/index.md, the antimycobacterial drugs in ../../5._Pharmacology/index.md, and the underlying organism in ../../6._Microbiology/index.md.

Practice Questions

Recall

Q: What staining property characterizes Mycobacterium tuberculosis, and why? A: It is acid-fast — its thick, mycolic-acid-rich cell wall retains carbol fuchsin dye even after acid-alcohol decolorization, so it appears as red rods on Ziehl-Neelsen staining rather than staining reliably by Gram method.

Understanding

Q: Why is TB treated with four drugs initially rather than one? A: Any large bacterial population contains rare spontaneous mutants resistant to a given drug. A single agent kills the susceptible majority but leaves resistant mutants to multiply. Using several drugs simultaneously means a mutant resistant to one is still killed by the others, preventing resistance and ensuring cure.

Application

Q: A patient on TB therapy reports his urine has turned orange-red. What is the cause, and what do you advise? A: Rifampicin causes harmless orange-red discoloration of body fluids (urine, tears, sweat). Reassure the patient it is expected and not dangerous; warn that it can stain soft contact lenses. This is not a reason to stop the drug.

Analysis

Q: A patient with HIV starts antiretroviral therapy shortly after beginning TB treatment and suddenly worsens with high fever and enlarging lymph nodes. What is happening and why? A: This is immune reconstitution inflammatory syndrome (IRIS): as antiretrovirals restore immune function, the recovering immune system mounts a vigorous inflammatory response against TB antigens, causing paradoxical clinical worsening despite effective treatment. It is managed supportively (sometimes with steroids) while continuing both therapies.

FAQ

Is everyone infected with TB going to get sick? No. Most people who inhale the bacteria develop latent infection and never become ill. Only about 5–10 percent of immunocompetent people progress to active disease over a lifetime; the risk is far higher with HIV or other immunosuppression.

Can TB be cured? Yes. Drug-susceptible TB is curable in the large majority of cases with six months of correctly taken combination therapy. The main threats to cure are poor adherence and drug resistance.

How infectious is TB, and when can a patient stop isolating? Only active pulmonary (and laryngeal) TB spreads, via airborne droplets from coughing. Patients typically become much less infectious after about two weeks of effective therapy with clinical improvement, though decisions depend on smear results and local guidelines.

Does the BCG vaccine prevent TB? BCG mainly protects infants and young children against severe forms such as TB meningitis and miliary TB. It gives limited and variable protection against adult pulmonary TB and can cause a positive skin test, which is why IGRA is preferred for latent-TB screening in vaccinated people.

Why does treatment take so long when other infections need only days? M. tuberculosis grows very slowly and includes dormant, slowly metabolizing bacteria that antibiotics kill inefficiently. Eradicating these persisters — and preventing relapse — requires months of therapy, not days.

Quick Revision

• TB is caused by acid-fast Mycobacterium tuberculosis, spread by airborne droplet nuclei.
• Latent TB: asymptomatic, non-infectious, positive TST/IGRA, normal imaging — treat to prevent reactivation.
• Active TB: cough more than 2–3 weeks, fever, night sweats, weight loss; upper-lobe cavities in reactivation.
• Diagnose active disease with sputum smear, culture (gold standard), and GeneXpert (rapid, detects rifampicin resistance).
• Standard therapy: RIPE for 2 months, then rifampicin + isoniazid for 4 months.
• Remember toxicities: isoniazid (neuropathy — give B6, hepatitis), rifampicin (orange fluids, interactions), pyrazinamide (hepatotoxic, gout), ethambutol (optic neuritis).
• MDR-TB = resistant to rifampicin + isoniazid; XDR adds fluoroquinolone + other resistance.
• HIV is the strongest risk factor for progression; screen every TB patient for HIV and vice versa.

Related Topics

Prerequisites

• The Medicine of Infections and How to Treat Them
• Microbiology of mycobacteria — ../../6._Microbiology/index.md

Related Topics

• Antimycobacterial pharmacology — ../../5._Pharmacology/index.md
• Granulomatous inflammation and caseous necrosis — ../../4._Pathology/index.md

Next Topics

• HIV and opportunistic infections — see the Infectious Diseases branch overview ../index.md
• Community control and screening programs — ../../8._Community_Medicine/index.md