English: Under a high magnification of 15549x, this scanning electron micrograph (SEM) depicted some of the ultrastructural details seen in the cell-wall configuration of a number of Gram-positive Mycobacterium tuberculosis bacteria. As an obligate aerobic organism, M. tuberculosis can only survive in an environment containing oxygen. This bacterium ranges in length between 2-4 microns, with a width of 0.2-0.5 microns. See PHIL 9997 for a colorized version of this image.

TB bacteria become active, and begin to multiply, if the immune system can't stop them from growing. The bacteria attack the body and destroy tissue. If in the lungs, the bacteria can actually create a hole in the lung tissue. Some people develop active TB disease soon after becoming infected, before their immune system can fight off the bacteria. Other people may get sick later, when their immune system becomes weak for another reason.

Babies and young children often have weak immune systems. People infected with HIV, the virus that causes AIDS, have very weak immune systems. Other people can have weak immune systems, too, especially people with any of these conditions: substance abuse; diabetes mellitus; silicosis; cancer of the head or neck; leukemia or Hodgkin's disease; severe kidney disease; low body weight; certain medical treatments (such as corticosteroid treatment or organ transplants); specialized treatment for rheumatoid arthritis, or Crohn's disease.