Mycobacterium tuberculosis, the micro-organism that causes tuberculosis, most commonly develops active infections in people whose immune systems are compromised and, therefore, unable to fight off the disease effectively. People suffering from reduced immune function from active HIV infection are particularly at risk, as are those with diabetes or alcoholism.
Mycobacterium tuberculosis spreads from person to person through respiratory secretions. The bacterium typically infects the lungs, where the body's defense responses surround the infection with scar tissue. This scar tissue results in the hard tubercules in the lungs that give the disease its name. People with compromised immune systems may not be able to prevent the disease from spreading through their lungs or to other parts of their bodies such as the kidneys or nervous system. In addition, latent bacteria from a previous exposure can reactivate in a person if the immune system becomes compromised at a later time.
Tuberculosis infection is a particular concern in hospital environments, where many individuals whose immune systems are compromised by disease or by the immunosuppressive effects of certain medical treatments and surgeries are in close proximity to one another. Tuberculosis is also easily spread anywhere people are in close proximity to one another. The lack of easily available treatment for tuberculosis in developing countries means that people in those countries are at higher risk of tuberculosis infection than those in developed countries.