Once uncommon in the United States, the number of tuberculosis (TB) cases starting growing again in the 1980s along with the spread of HIV as the two diseases spurred each other on. Tuberculosis remains a major cause of sickness and death in less developed parts of the world, particularly Asia and Africa. Although the Merck Manual estimates that about one-third of the world's population carries tuberculosis, only about 15 million people have active and contagious infections at any given time.
Contagion
Only people with active tuberculosis infections in the lungs, whose sputum contains a high number of particles, can spread TB. People who have pits in their lungs cause by TB, called cavitary TB, are especially contagious. Once patients take medication to control TB for at least two weeks, they cough much less and the number of bacterial particles declines so that they are no longer contagious.
Exposure
The Merck Manual says that infection with TB almost always occurs through inhaling airborne bacterial particles. Coughing, singing or anything that forces air and sputum up from the lungs of an actively infected person contaminates the air. The airborne bacteria stay suspended in the air for up to several hours. Once they land on a surface, though, they are unlikely to be resuspended and inhaled. To catch TB, the exposed person must breathe the bacterial particles deep into the lungs, where they must be gobbled up by immune cells called macrophages. If a macrophage does not kill the bacteria, the bacteria multiplies itself, kills the macrophage and spreads.
Environment
Factors that favor frequent or extended exposure to an actively infected person increase the likelihood of TB infection. People living in poorly ventilated or over-crowded spaces have an increased risk, as do doctors, nurses and health care workers who come into close contact with actively infected patients.
Infection
Although a person with an active TB lung infection spreads the disease to about seven other people on average, according to the Merck Manual, most of those exposed do not develop the disease themselves. In most cases, the immune system controls the infection and suppresses it in a latent state. Only 10 percent of people with a healthy immune system go on to develop active TB after latent infection, says the Merck Manual. The Mayo Clinic says that the risk of active infection is highest in the first two years following exposure to the tuberculosis bacteria. People with latent TB cannot spread the disease.
At-Risk Populations
The elderly, patients with impaired immune systems--due to HIV/AIDS, in particular--patients taking immunosuppressant drugs, those with diabetes, kidney disease, head or neck cancer, people under a lot of stress or those who recently had some type of abdominal surgery are particularly at risk for developing active TB after exposure. The same patients risk development of tuberculosis outside of the lungs, which is more difficult to cure.
