Shadows of children infected with tuberculosis (TB) are seen on the ground during a march in New Delhi to mark World Tuberculosis Day in 2006. TB microbes spread from person to person through the air. Globally, nearly 9 million people get TB each year, and 1.6 million die as a result. One-third of the world population is infected with latent (inactive) TB.

The disease usually does not become active unless something reduces a person's immunity -- a disease like AIDS, advancing age or some medical conditions. TB is treated with a six- to nine-month course of "first-line" (most effective) drugs. If patients do not complete the drug course or are not treated properly, they can develop MDR-TB, which is resistant to at least two of the best anti-TB drugs, isoniazid and rifampicin. (© AP Images)