Tuberculosis - Causes

The most common method by which TB is transmitted is coughing or sneezing. When a person coughs or sneezes, he or she releases a fine mist of water droplets. If the person carries the tubercle bacillus, those droplets may contain thousands of the bacteria. A person nearby may inhale those water droplets and the bacteria they contain. The bacteria can then travel to that person's respiratory system and cause a new infection.

About a third of the people standing close to a person with TB are likely to develop the disease. Tuberculosis is not transmitted by contact with a person's clothing, bed linens, or dishes and cooking utensils. A fetus may become infected, however, by taking in bacilli from the mother.


The tubercle bacilli a person inhales may or may not cause tuberculosis. The human immune system has a variety of ways to capture and kill these bacteria. If the immune system is successful in doing so, the person will not become ill with TB.

Inhaled bacilli, however, may survive the immune system. They may travel throughout the body to organs other than the lungs. In some cases, the bacilli remain active enough to cause tuberculosis. In about 5 percent of all cases, a person develops tuberculosis within twelve to twenty-four months of being exposed to TB bacteria.

Bacillus Calmette-Guérin (BCG):
A vaccine made from weakened mycobacterium that infects cattle. It is used to protect humans against pulmonary tuberculosis and its complications.
Outside of the lungs.
Any change in the structure or appearance of a part of the body as the result of an injury or infection.
Mantoux test:
Another name for the PPD test, which is used to determine whether a person has been infected with the tuberculosis bacterium.
Miliary tuberculosis:
A form of tuberculosis in which the bacillus spreads throughout the body producing many thousands of tubercular lesions.
A group of bacteria that includes Mycobacterium tuberculosis, the bacterium that causes tuberculosis.
the membrane surrounding the heart.
Relating to the lungs.
Purified protein derivative (PPD):
A substance injected beneath the skin to see whether a person presently has or has ever had the tubercle bacillus.
Secretions produced inside an infected lung. When the sputum is coughed up it can be studied to determine what kinds of infection are present in the lung.
A substance that causes the body's immune system to build up resistance to a particular disease.

By contrast, less than 10 percent of all people who inhale the tubercle bacillus actually become ill. The rest develop no symptoms of the disease and have negative X rays for the disease. In such cases, the disease is said to be inactive. The bacilli remain alive in cells, but they are not active enough to actually cause disease. They may become more active later in life, however.

In such cases, a person may become ill with tuberculosis long after being exposed to the TB bacteria.

Scientists believe that anywhere from ten to fifteen million Americans are carrying inactive tubercle bacilli in their bodies.

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