Tuberculosis (TB) is a serious respiratory (lung) disease that can be life-threatening if not treated correctly. TB is, in fact, the world's most common disease caused by an infectious organism. Nearly 2 billion people in the world are diagnosed with TB every year, a disease that is also responsible for the deaths of nearly 3 million people annually.
In industrialized nations such as the United States, TB was well on its way to becoming extinct 15 years ago. With the HIV epidemic, however, TB rates started increasing again between 1985 and 1992. Since 1992, the total number of TB cases has once again decreased. However, in certain groups of people in the U.S.—such as people immigrating to the United States from countries where TB rates are very high—the TB rate is increasing. In 2000, there were 17,531 cases of TB. Although the number of TB cases continues to decrease, it remains one of the most common causes of sickness and death in U.S. residents infected with HIV. In fact, TB is the number-one cause of death of HIV-infected people across the globe.
Mycobacterium tuberculosis, the bacteria that causes TB, is spread from one person to another. Using microscopic drops of fluid produced by the lungs, the bacteria can travel from the lungs of an infected person and be deposited in the lungs of someone nearby. Once inside the lungs, the bacteria establishes infection. Even though 150,000 people in the United States have been infected with this bacteria, most people (between 90% and 95%) have immune systems that are healthy enough to prevent the bacteria from ever causing TB. In people with HIV, the immune system may eventually lose control of the bacteria, causing the infection to spread and cause active disease. This process can take many months or years. In other words, Mycobacterium tuberculosis can remain alive in someone's body for many years, but may only become active—i.e., cause tuberculosis—once the immune system becomes damaged.
Tuberculosis almost always causes disease of the respiratory system. In HIV-positive people, particularly those with CD4 cell counts below 200, the bacteria can also infect the lymphatic system (i.e., the lymph nodes and the spleen).
Compared to HIV-negative patients with TB, HIV-positive people with the disease may see their symptoms develop faster and with greater intensity. Treating TB in HIV-infected patients may also need to be more aggressive in order to clear the bacteria from the body.