It is estimated that up to one third of the world's population may be infected with the tubercule bacillus. Most of these people, however, do not have active TB. Their infection is latent, and may remain so for their whole life, not causing any symptoms.

In a proportion of people, however, the infection becomes active and begins to destroy the tissues of the organ it has invaded.

Most commonly this is the lungs but TB can also be extrapulmonary, affecting other parts of the body including the spine (in which case it is called Pott's disease), the kidneys, the meninges (which are the membranes covering the brain), and the lymph nodes. In general the infection progresses slowly.

If affected persons do not receive treatment, roughly one third of those with active TB die within two years and another third within five years. The epidemiological significance of this long period between onset of symptoms and death is that during all this time the affected person is coughing and spreading the TB bacillus to others in his or her entourage.

The World Health Organisation (WHO) estimates that one person becomes infected with TB every second. The TB bacillus causes a process called caseation, a word derived from the Latin for cheese. The slowly progressive inflammation of TB destroys the tissues and leaves in their place a thick cheesy substance.

Especially if more than one organ is affected this leads the person to lose weight in a dramatic fashion, almost as if they are being consumed by the disease. This, indeed, gave TB its common name during the 19th century, consumption. The final cause of death may be either multiple organ failure or internal haemorrhage (bleeding) in the lungs, which may happen when the advancing destruction erodes into an artery.