Tuberculosis is an airborne disease caused by an organism called Mycobacterium tuberculosis. When a person who is infectious (meaning he/she can transmit the disease to someone else) coughs or sneezes, droplet nuclei are expelled into the air. A person who breathes in these particles over a long period of time may become infected. Once infected, a person does not necessarily develop the disease. Infected persons will show up positive on a PPD (the skin test for tuberculosis), but the immune system can keep the disease under control by producing special immune cells which wall off the infection and prevent further spreading.

When a PPD is positive, the only way to be sure an individual does not have the disease is by further medical evaluation. A physician will look for symptoms of pulmonary disease, which include coughing, pain in the chest when breathing or coughing, and coughing up sputum (phlegm from deep in the lungs) or blood, and general symptoms, such as weight loss, fatigue, malaise, fever, and night sweats, which can be from pulmonary or extrapulmonary disease. A chest x-ray and bacteriologic examinations are needed to further confirm and diagnose a case of TB disease.

People who are HIV-positive will be considered PPD positive if the test results in swelling measuring 5 or more millimeters, in contrast to 15 millimeters for people with no risk factors. (Redness is not measured, as redness is not a sign of TB infection.) A person who is HIV-positive and has a severely suppressed immune system may not react to the PPD, even though he/she is infected. This is called anergy. Some who have been exposed to TB will be given preventive therapy even without a positive PPD.

An individual with a healthy immune system has a 10% chance over his lifetime of developing TB disease once he is infected. An individual who is HIV-positive, however, is more at risk, with an 8 to 10% chance per year of developing TB disease once he is infected. Individuals who are HIV-positive are given a high priority to receive preventive therapy, usually with isoniazid (INH). A person with a healthy immune system and not infected with HIV needs 6 months of preventive therapy, while an HIV-infected individual needs to be on preventive therapy for 12 months.

It is extremely important to continue to take the prescribed medicine for the time period recommended, or drug resistance can occur. If an individual stops taking the medicine halfway through the course of treatment, and then develops the disease at a later time, the TB germ may have built up a resistance to that medication, making it useless in the treatment of that individual’s disease. The two best medications we have to treat tuberculosis are INH and rifampin. If a person is resistant to at least both of these medications, she is said to be "multidrug-resistant" (MDR), and she has a 50 to over 80% chance of being cured, rather than the 95% expected cure rate of TB which is sensitive to all medications.

A patient who is multidrug-resistant may have to receive treatment for up to several years in order to achieve cure. The other bad news about multidrug-resistant TB is that if I catch this strain from someone, then I, too, am multidrug-resistant, and I have a 50 to over 80% chance of cure instead of 95% chance, in spite of having no risk factors.