How is TB transmitted?

When an adult with infectious TB coughs, sneezes, sings or talks, the TB bacteria may be expelled into the air in the form of small particles called droplet nuclei, which cannot be seen except through a microscope. Transmission occurs when a person in close contact inhales (breathes in) the droplet nuclei.

Figure 13.2 shows an infectious TB patient expelling a large amount of droplet nuclei after coughing, and those nuclei being inhaled by a nearby person. If an infectious adult spits indiscriminately, the sputum containing bacteria dries and wind can carry the droplet nuclei into the air, so anyone can inhale them.

In addition, consumption of raw milk containing Mycobacterium bovi (TB bacteria found in domestic animals such as cows, goats and lambs) may also cause TB in humans, though nowadays it is much less frequent because of boiling milk or pasteurisation (the processing of removing germs from milk).

The contact person does not usually develop active TB immediately. In some cases, the person's immunity is able to remove the bacteria and he/she does not develop TB infection. In other cases, the person develops an immune response that controls the bacteria by 'walling it off' inside the body. This causes the bacteria to become inactive. The person does not develop active TB or become ill at the time, but is said to have latent tuberculosis infection (LTBI). Up to one-third of the world's population is thought to be infected with latent TB.

If the immunity of a person with LTBI is weakened, the body is no longer able to contain the TB bacteria, which then grow rapidly and the person becomes sick with symptoms and signs of TB. The person is then said to have active TB. This process of progression from LTBI to active TB is called reactivation. The greatest risk for developing active TB is within the first two years following the initial infection.