An ancient, patient disease
Tuberculosis (TB) has walked with humanity for thousands of years — traces of it appear in Egyptian mummies. It is caused by a single, remarkable germ: Mycobacterium tuberculosis. Most often it settles in the lungs, but it can reach almost any organ. What makes TB so different from a common chest infection is its patience: it can sit quietly in the body for decades before ever causing illness, or never cause illness at all.
Despite being treatable and curable, TB remains one of the world's leading infectious killers, claiming more than a million lives a year. The good news is that the science is well understood, the tests are clear, and the cure — though long — works when it is finished properly.
A bug wrapped in wax
The secret to TB's behaviour lies in its cell wall. M. tuberculosis is coated in a thick, fatty, waxy layer rich in mycolic acids. This greasy armour does three things: it makes the bug grow extremely slowly (dividing once every 15–20 hours, versus every 20 minutes for many ordinary bacteria), it helps it survive inside our own immune cells, and it makes it hard to stain with the usual laboratory dyes.
Because the wax resists being washed out by acid after staining, the bug earns its laboratory name: the acid-fast bacillus. We will see in a later guide how this single property became the cheapest and oldest TB test in the world.
What the body does in reply
When inhaled bacteria reach the deep lung, the first cells to meet them are the alveolar macrophages — the lung's resident clean-up cells. Macrophages normally swallow and digest invaders. But M. tuberculosis can survive inside them, turning the macrophage into a hiding place. The immune system answers by walling the infection off.
Immune cells pile up around the infected macrophages to form a tiny, organised ball called a granuloma — the signature lesion of TB. Think of it as a containment fortress: it traps the bacteria inside, often holding them in check for life. The granuloma is the physical reason TB can become latent rather than active, the distinction we explore in the next guide.