Tuberculosis (TB) Facts (cont.)

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Are there different types of tuberculosis (TB)?

There are many types of tuberculosis, but the main two types are termed either active or latent TB. Active TB is when the disease is actively producing symptoms and can be transmitted to other people; latent disease is when the person is infected with Mycobacterium tuberculosis bacteria, but the bacteria are not producing symptoms (usually due to the body's immune system suppressing the bacterial growth and spread). People with latent TB usually cannot transfer Mycobacterium tuberculosis bacteria to others unless the immune system fails; the failure causes reactivation (bacterial growth is no longer suppressed) that results in active TB so the person becomes contagious.

Many other types of TB exist in either the active or latent form. These types are named for the signs and for the body systems Mycobacterium tuberculosis preferentially infect, and these infection types vary from person to person. Consequently, pulmonary TB mainly infects the pulmonary system, cutaneous TB has skin symptoms, while miliary TB describes widespread small infected sites (lesions or granulomas about 1 mm-5 mm) found throughout body organs. It is not uncommon for some people to develop more than one type of active TB. More types will be listed in the symptoms and signs section below.

What causes tuberculosis?

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The cause of TB is infection of human tissue(s) by the bacterium Mycobacterium tuberculosis (mycobacteria). These bacteria are slow growing, aerobic, and can grow within body cells (an intracellular parasitic bacterium). Its unique cell wall helps protect it from the body's defenses and gives mycobacteria the ability to retain certain dyes like fuschsin (a reddish dye) after an acid rinse that rarely happens with other bacterial, fungal, or parasitic genera.

Mycobacteria that escape destruction by body defenses may be spread by blood or lymphatic pathways to most organs, with preference to those that oxygenate well (lungs, kidneys, and bones, for example). Typical TB lesions, termed granulomas, usually consist of a central necrotic area, then a zone with macrophages, giant Langerhans cells and lymphocytes that become surrounded by immature macrophages, plasma cells, and more lymphocytes. These granulomas also contain mycobacteria. In latent infections, a fibrous capsule usually surrounds the granulomas, and in some people, the granulomas calcify, but if the immune defenses fail initially or at a later time (reactivate), the bacteria continue to spread and disrupt organ functions.

Medically Reviewed by a Doctor on 1/24/2014

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