Overview

In developed countries, such as the United States, many people think tuberculosis (TB) is a disease of the past. TB, however, is still a leading killer of young adults worldwide. Some 2 billion people—one-third of the world's population—are thought to be infected with TB bacteria, Mycobacterium tuberculosis.

TB is a chronic bacterial infection. It is spread through the air and usually infects the lungs, although other organs and parts of the body can be involved as well. Most people who are infected with M. tuberculosis harbor the bacterium without symptoms (have latent TB), but some will develop active TB disease. According to World Health Organization (WHO) estimates, each year, 8 million people worldwide develop active TB and nearly 2 million die.

One in 10 people who are infected with M. tuberculosis may develop active TB at some time in their lives. The risk of developing active disease is greatest in the first year after infection, but active disease often does not occur until many years later.

TB in the United States

In 2005, the Centers for Disease Control and Prevention (CDC) reported 14,093 cases of active TB. While the overall rate of new TB cases continues to decline in the United States since national reporting began in 1953, the annual decrease in TB cases has slowed from an average of 7.1 percent (1993–2000) to the current average of 3.8 percent (2001–2005), according to CDC. In addition to those with active TB, an estimated 10 to 15 million people in the United States have latent TB.

Minorities are affected disproportionately by TB, which occurs among foreign-born individuals nearly nine times as frequently as among people born in the United States. This is partially because they were often exposed to M. tuberculosis in their country of origin before moving to the United States. In 2004, a very high percentage of Asians (95 percent) and Hispanics (75 percent) who were born outside the United States were reported to have TB.

Cause

Bacterium

Tuberculosis is caused by the infectious agent known as Mycobacterium tuberculosis (Mtb). This rod-shaped bacterium, also called Koch's bacillus, was discovered by Dr. Robert Koch in 1882.

Characteristics

• Mtb is a small, slow-growing bacterium that can live only in people; it is not found in other animals, insects, soil, or other nonliving things.
• Mtb is an aerobic bacterium, meaning it needs oxygen to survive. For this reason, during active TB disease, Mtb complexes are always found in the upper air-sacs of the lungs.

TB Infection

When a person breathes in Mtb-contaminated air, the inhaled TB bacteria reach the lungs. This causes an Mtb infection. However, not everyone infected with TB bacteria becomes sick. The bacteria can remain dormant (asleep) for years and not cause any TB disease. This is called latent TB infection. People who have latent TB infection do not get sick and do not spread the bacteria to others. But, some people with latent TB infection eventually do get TB disease.

So it is important to get the appropriate treatment and get rid of the bacteria even in latent TB infection.

TB Disease

For someone to develop active TB disease, the following two events must take place:

1. The bacteria enter the body and cause an Mtb infection.
2. The immune system cannot stop the TB bacteria from growing and spreading after the initial infection.

One in ten people infected with TB bacteria develop active TB disease at some point in their lives. The active bacteria multiply and destroy the tissue. A person with TB disease shows symptoms that vary, depending on where the TB bacteria are growing. In most cases, the bacteria attack the lungs.

TB is primarily an airborne disease. The bacteria are spread from person to person in tiny microscopic droplets when a TB sufferer coughs, sneezes, speaks, sings, or laughs. Only people with active TB can spread the disease to others. People with TB who have been treated with the correct drugs for at least 2 weeks, however, are no longer contagious and do not spread the bacteria to others.

Symptoms

Early symptoms of active TB can include weight loss, fever, night sweats, and loss of appetite. Symptoms may be vague, however, and go unnoticed by the affected person. For some, the disease either goes into remission (halts) or becomes chronic and more debilitating with cough, chest pain, and bloody sputum.

Symptoms of TB involving areas other than the lungs vary, depending upon the organ or area affected.