(Source: NIH)

What is Tuberculosis?
Tuberculosis (TB) is an often severe and contagious airborne disease caused by infection with Mycobacterium tuberculosis (Mtb). TB typically affects the lungs but it also may affect any other organ of the body. It is usually treated with a regimen of drugs taken for six months to two years depending on the type of infection.

For someone to develop active TB disease, the following two events must take place:

The bacteria must enter the body, thereby causing an Mtb infection. This usually happens when a person breathes in Mtb-contaminated air and the inhaled TB bacteria make their way to the lungs.
The immune system cannot stop the TB bacteria from growing and spreading after the initial infection.
The Difference Between Mtb Infection and TB Disease
In understanding tuberculosis, it’s important to understand the difference between Mtb infection and TB disease.

Mtb Infection
It is estimated that more than one-third of the world’s population has the TB bacterium in their bodies, which means they have Mtb infection. In addition, new infections are occurring at the rate of one per second. Fortunately, only a fraction of people infected with Mtb develops active TB disease. Those who do not get sick are known to have non-contagious latent TB, so-called because the bacteria are inactive or “asleep” in the body.

TB bacteria can remain in this dormant state for months, years, and even decades without increasing in number and without making the person sick. Most people with latent Mtb infection will test positive on the tuberculin skin test, or their chest X-ray will show signs of latent TB. These findings indicate that they have the TB germ in their bodies, but most infected people will not develop active TB disease, may never get sick, may never show any symptoms, and may never spread the bacteria to others.

However, one in ten people infected with TB bacteria, do develop active TB disease. Therefore, it is important to get the appropriate treatment and get rid of the bacteria in both latent TB and active TB disease.

TB Disease
People with weakened immune systems (individuals with HIV disease, those receiving chemotherapy, or children under five years of age, for example) are at a greater risk for developing TB disease. When they breathe in TB bacteria, the bacteria settle in the lungs and start growing because the individual’s immune system cannot fight the bacteria. In these people, TB disease may develop within days or weeks after the infection. However, in some other people, TB disease may develop months or years after the initial infection, at a time when the immune system becomes weak for other reasons and is no longer able to fight the bacteria.

When a person gets active TB, it means the TB bacteria are multiplying and attacking the lung(s), or other parts of the body such as the lymph nodes, bones, kidney, brain, spine, and even the skin. From the lungs, the TB bacteria move through the blood to different parts of the body. Symptoms of active disease include cough, loss of weight and appetite, fever, chills and night sweats as well as symptoms from the specific organ or system that is affected; for example, coughing up blood or sputum in TB of the lungs, or bone pain if the bacteria have invaded the bones.

TB disease usually can be cured with prompt and appropriate treatment, but it remains a major cause of death and disability in the world, particularly among persons infected with human immunodeficiency virus (HIV).

TB Bacteria are Spread Only from a Person with Active TB Disease
In people who develop active TB of the lungs, also called pulmonary TB, the TB skin test will often be positive. In addition, these people will show all the signs and symptoms of TB disease, and can pass the bacteria to others. So, if the patient with TB of the lungs, sneezes, coughs, talks, sings or does anything that forces the bacteria into the air, other people nearby may breathe in the TB bacteria. Statistics show that approximately one-third of the people exposed to pulmonary TB become infected with the bacteria, but only one in ten of these infected people develop active TB disease during their lifetimes.

Among people suffering from TB disease, three out of four have disease affecting the lungs. If not treated immediately, the bacteria have the potential to destroy the lungs and kill the person.

Multidrug-Resistant Tuberculosis (MDR TB)
MDR TB is a form of drug-resistant TB in which the TB bacteria can no longer be killed by at least the two best antibiotics, isoniazid (INH) and rifampin (RIF), commonly used to cure TB. As a result, this form of the disease is more difficult to treat than ordinary TB and requires up to two years of multidrug treatment.

People may get MDR TB in two ways:

directly, if they spend time with an MDR TB patient and breathe in the MDR TB bacteria, and
if they already have active TB and do not properly follow their prescribed treatment regimen or TB medication is not reliably available to them. This inconsistent use of TB antibiotics gives the bacteria enough time to evolve and evade the first-line anti-TB medications, and regular TB may then progress to MDR TB, which is more challenging to treat.
Extensively Drug-Resistant Tuberculosis (XDR TB)
XDR TB is a less common form of multidrug-resistant TB in which the TB bacteria have changed enough to circumvent the two best antibiotics, isoniazid (INH) and rifampin (RIF), as well as most of the alternative drugs used against MDR TB. These second-line drugs include any fluoroquinolone, and at least one of the other three injectable anti-TB drugs: amikacin, kanamycin, or capreomycin. As a result, this form of the disease needs up to two years of extensive drug treatment and is the most challenging to treat.

People may get XDR TB in two ways:

directly, if they spend time with an XDR TB patient and breathe in the XDR TB bacteria, and
if they already have MDR TB or active TB, and do not properly follow their prescribed treatment regimen or TB medication is not reliably available to them. This inconsistent use of TB antibiotics gives the bacteria enough time to evolve and evade most if not all TB drugs, making it extremely difficult or impossible to treat XDR TB.