How it travels: the airborne route
TB spreads through the air. When a person with active lung TB coughs, sings, or even speaks, they release tiny droplets carrying the bacteria. The water evaporates, leaving droplet nuclei so light they can float in a room for hours. Someone who breathes them in may inhale the germ all the way down to an alveolus in the deep lung.
TB is contagious but not easily caught from a passing encounter. It usually takes prolonged, close, indoor contact — sharing a household or a poorly ventilated room for many hours. It does not spread by touching surfaces, sharing dishes, or shaking hands. Fresh air and sunlight quickly destroy the floating germs, which is why crowding and poor ventilation are the real drivers of transmission.
The first encounter: primary infection
The very first time the body meets TB is called primary TB. The germ lands in the lung, the immune system rallies, and a small scarred lesion forms — the Ghon focus — often along with a swollen lymph node at the hilum. In most healthy people this primary battle is won quietly, with no symptoms at all. A few surviving bacteria may simply be sealed inside granulomas.
If disease later flares up — sometimes years afterward, often in the upper lungs where oxygen is plentiful — we call it post-primary TB (also called reactivation TB). This is the cavity-forming, infectious form most people picture when they think of consumption.
The big distinction: latent vs active
This is the single most important idea in all of TB. Latent TB infection means the bacteria are present but walled off and dormant. The person feels completely well, has a normal chest X-ray, and — crucially — cannot infect anyone. Roughly a quarter of the world's population carries latent TB. Most will never become ill.
Active tuberculosis is the opposite: the bacteria have broken free and are multiplying and damaging tissue. The person is sick and, if it is in the lungs, contagious. The lifetime risk of latent TB progressing to active disease is roughly 5–10%, but it rises sharply when the immune system is weakened — by HIV, malnutrition, diabetes, or certain medications. The whole purpose of treating latent TB is to stop that quiet infection from ever becoming active.