All cases of TB are passed from person to person via
droplets. When someone with TB infection coughs, sneezes, or talks, tiny
droplets of saliva or mucus are expelled into the air, which can be inhaled by another person.
- Once infectious particles reach the alveoli (small saclike structures in the air spaces in the lungs), another cell, called the macrophage, engulfs the TB bacteria.
- Then the bacteria are transmitted to the lymphatic system and bloodstream and spread to other organs occurs.
- The bacteria further multiply in organs that have high oxygen pressures, such as the upper lobes of the lungs, the kidneys, bone marrow, and meninges -- the membrane-like coverings of the brain and spinal cord.
- When the bacteria cause clinically detectable disease, you have TB.
- People who have inhaled the TB bacteria, but in whom the disease is controlled, are referred to as infected. Their immune system has walled off the organism in an inflammatory focus known as a granuloma. They have no symptoms, frequently have a positive skin test for TB, yet cannot transmit the disease to others. This is referred to as
latent tuberculosis infection or LTBI.
- Risk factors for TB include the following:
- HIV infection,
- low socioeconomic status,
- crowded living conditions,
- diseases that weaken the immune system,
- migration from a country with a high number of cases,
- and health-care workers.
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