Tuberculosis, an airborne disease caused by Mycobacterium tuberculosis, primarily affects the lungs though it can spread to other organs. More than 13,000 new cases of tuberculosis, or TB, were reported in the United States in 2007, according to Merck Manuals Online Medical Library. Tuberculosis affects the lungs tissues in many ways, depending on the severity of the disease.
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In latent TB, a dormant form of tuberculosis, bacteria breathed into the lungs are surrounded by white blood cells. If the white blood cells, called macrophages, contain the infection, bacteria remain walled off in areas called granulomas and active infection doesn’t develop. Small scars appear in the lungs where the bacteria are walled off. Immune substances released from the soft, crumbly center kill off most of the bacteria, although some may remain.
If the granuloma ruptures and leaks fluid into the space between the lung and the chest wall, called the the pleural cavity, tuberculosis pleurisy can develop. Fluid within the space increases, causing shortness of breath and chest pain that worsens when the person breathes in. Most cases resolve spontaneously but about two-thirds will develop active tuberculosis pleurisy within five years, Healthcommunities.com reports.
The bacteria in the lungs may re-activate if the immune system is damaged by diseases such as alcoholism or malnutrition, by treatments such as chemotherapy or by prolonged use of medications such as corticosteroids that cause immune suppression. Advanced age can also impair the immune system and cause re-activation of latent TB. When this happens, granuloma starts to break down and liquefied material escapes in the airway. A cavity forms in the lung, which allows oxygen and carbon dioxide to enter. Since these provide an excellent medium for bacterial growth, the TB bacteria reproduce rapidly. Cavity formation causes destruction of lung tissue, with coughing, spitting up blood, fever, night sweats and weight loss. People with cavitary TB are very contagious, Healthcommunities warns.
In miliary TB, small nodules that look like millet seeds form throughout the lung shortly after the initial infection. The chest X-ray may initially appear normal, making diagnosis difficult. Miliary TB is a serious form of TB that can result in death.