Tuberculosis

Tuberculosis, or TB, is caused by the bacterium Mycobacterium tuberculosis. It spreads when someone with active lung TB coughs, sneezes or speaks, releasing tiny droplets that others breathe in. The bacteria are slow-growing rods whose waxy cell wall, rich in mycolic acids, makes them tough to kill and gives them their acid-fast staining. TB usually attacks the lungs, but it can also affect the lymph nodes, kidneys, spine and brain.

Once inhaled, the bacteria are swallowed by macrophages in the alveoli but survive and multiply inside them. In most people the immune system walls the infection off in clusters of immune cells called granulomas, or tubercles. This latent infection causes no symptoms and cannot be passed on. About a quarter of the world's population is thought to carry it, and only a small share, roughly 5 to 10 percent, ever develop active disease, often when immunity weakens, as with HIV. Active TB brings a cough lasting weeks, sometimes with blood, along with fever, night sweats and weight loss.

Skin or blood tests reveal infection, while chest X-rays, sputum smears, culture and rapid molecular tests confirm active disease. Treatment combines several antibiotics, classically isoniazid, rifampin, pyrazinamide and ethambutol, taken for months, and drug-resistant strains need longer regimens. The BCG vaccine protects young children against severe forms.