TB: BIOLOGICAL ISSUES

What is TB?

Tuberculosis (TB) is a disease caused by bacteria Mycobacterium tuberculosis. The vast majority of people who develop clinical TB develop disease in their lungs (pulmonary TB, or you may see it written as PTB). A small percentage of TB cases occur outside of the lungs, and this is called extra-pulmonary TB. Extra-pulmonary TB can occur in other places throughout the body, including the bowel, joints, throat, and pleura.

When M. tuberculosis enters the lungs and interacts with the body’s immune system, it has one of four fates. The body’s immune system can kill the bacteria immediately (eradication), it can suppress the bacteria and keep it dormant (latent TB), it can fail to manage the bacteria and disease can develop (active TB), or, for a number of reasons, the immune system can eventually deteriorate and latent TB can become active (reactivation).

What is the difference between latent TB and active TB?

Latent TB means that you have been exposed to TB but your immune system is strong enough to keep it walled off. If you have latent TB, you are not considered infectious, and are not considered to have clinical TB. You will, however, produce a positive test for tuberculosis if being tested by the common skin test (PPT). Active TB is clinical “TB disease.” Latent TB does pose the risk of developing into active TB if the immune system becomes compromised (i.e. old age, diabetes, HIV). HIV negative individuals have about a 1 in 10 chance of developing active TB throughout their lifetime. HIV positive individuals have about a 1 in 10 chance of developing active TB disease each year.

The difference in active and latent TB becomes important when you read statistics on TB. When medical and public health professionals refer to “cases” of TB they are referring to active TB disease.

How is TB spread?

TB is one of the few truly airborne pathogens. The bacteria is spread when a susceptible individual inhales infectious aerosolized droplets generated when someone with active TB coughs, sneezes, or otherwise expels air with moderate force (i.e. singing to a lesser extent). The droplets are tiny, only 1-5 µm (0.001-0.005 mm) in diameter. This size is critical for two primary reasons. First, the small size allows the bacteria to remain suspended in air for long periods of time. Thus, air may be infectious even after the infectious person leaves the room. Second, the size allows the bacteria to penetrate deep into the lower lung and avoid biological defenses of the human body.

TB can not be spread by holding hands, touching doorknobs, etc.