Tuberculosis

Overview

Tuberculosis (TB) is an infectious disease caused by a bacterium called Mycobacterium tuberculosis. The infection most often affects the lungs but may involve any organ. TB is spread from person to person, usually through the air, when a person with active disease coughs and sprays the bacteria into the air.

Not everyone who inhales the germ develops active TB disease. In most tuberculosis infections, the body's natural defenses are able to control the infection. Only about 10 percent of those infected will develop active disease in their lifetimes. Instead, the bacteria persist as a latent TB infection, which cannot be spread to other people. Active disease can occur in an infected person when the body's resistance is low or if a large or prolonged exposure to the germs overcomes the body's natural defenses. The body's response to active TB infection produces inflammation that can damage the lungs. The damage can be extensive even if the symptoms are minimal.

In the United States, TB is much less common than it used to be. Of the some 14,000 new cases of active disease each year in the United States, most are either related to HIV/AIDS, homelessness, and drug abuse or occur in people who were born abroad. Tuberculosis is very common in the developing world. It has been estimated that as much as a third of the world's population is infected with tuberculosis, and worldwide about 1.6 million people die of TB every year. TB and HIV are closely associated; people with HIV are much more likely to develop active disease if they are infected with the bacteria that cause TB.

Since the introduction of effective antibiotics, tuberculosis management has changed dramatically. Most important, people with tuberculosis are no longer sent to specialized sanitariums; now, they are treated in general hospitals and clinics. Also, doctors now know that they can reliably prevent active disease among people with latent infections. However, misuse of drugs has led to the development of drug-resistant tuberculosis, which is harder to cure. If antibiotics don't work, TB can be deadly.

This section contains more information on:
• Need-to-Know Anatomy
• Causes
• Risk Factors
• Active vs. Latent TB
• Links

Need-to-Know Anatomy

Active tuberculosis disease occurs most often in the lungs. The lungs are where your body carries out gas exchange between the air and the bloodstream, exchanging carbon dioxide for fresh oxygen. This exchange takes place at the alveoli, tiny little bubblelike divisions of the lungs. When you breathe, you inhale plenty of other material along with oxygen, including bacteria. In order to infect you, the tuberculosis bacteria have to pass through the defenses in your airway and reach the alveoli.

When the body's immune system notices the bacteria, it surrounds them with immune cells, making blobs known as granulomas and effectively walling them off from the rest of the body. These granulomas are harmless. The bacteria can persist in granulomas for many years as a latent TB infection.

Causes

Tuberculosis is caused by a bacterium known as Mycobacterium tuberculosis. (The related bacteria Mycobacterium bovis and Mycobacterium africanum can also cause tuberculosis, but this is rare.) The body's response to active TB infection produces inflammation that can damage the lungs. Areas affected by active tuberculosis gradually fill with scar tissue.

Risk Factors

You are at risk of tuberculosis infection if you are around people with active TB disease who are coughing, which releases bacteria into the air. The risk of infection increases for intravenous drug users, healthcare workers, and people who live or work in a homeless shelter, migrant farm camp, prison or jail, or nursing home.

Most people who are infected with the bacteria that cause TB do not develop active disease. The following factors raise the risk that latent disease will develop into active disease:

• infection with HIV, the virus that causes AIDS and weakens the immune system
• diabetes
• low body weight
• head or neck cancer, leukemia, or Hodgkin's disease
• some medical treatments, including corticosteroids or certain medications used for autoimmune or vasculitic diseases such as rheumatoid arthritis or lupus, which suppress the immune system.
•silicosis, a respiratory condition caused by inhaling silica dust.

Active vs. Latent TB

Active TB disease is an illness where the TB bacteria are multiplying and attacking different parts of your body, most often the lungs. A person with active TB disease can spread TB to others. If you are diagnosed with an active TB disease, be prepared to give a careful, detailed list of every person you have had contact with so those people can be tested for TB as well.

Miliary TB is a rare form of active disease that occurs when tuberculosis bacteria find their way into the bloodstream through an open wound. In this form, the bacteria quickly spread all over the body in tiny nodules and affect multiple organs at once. This can be rapidly fatal.

Latent TB infection is less aggressive than the active form of the disease. A latent TB infection means that your body is infected with live TB germs, but they are not rapidly reproducing and spreading. If you have a latent TB infection, your skin test will usually be positive but you will have no symptoms: You won't feel sick, and you can't spread TB to others. However, you may develop active TB disease if you do not receive treatment for latent TB infection and your immune system can no longer keep the bacteria in check.

Links

More information on tuberculosis is available at these websites recommended by the U.S.News & World Report library:

American Lung Association
The American Lung Association provides a basic overview of what TB is, what the symptoms are, who gets it, and how it's treated. It offers newsletters on lung health via E-mail, a hotline for questions, and a searchable database of local chapters for support.

Clinical Trials
The U.S. National Institutes of Health provides a list of clinical trials currently recruiting patients as well as those in the planning stages.

Medline: Tuberculosis
The National Library of Medicine's guide to TB includes the latest news on studies, diagnosis and treatment information, a link to state TB control offices, and much more.

National Institute of Allergy and Infectious Diseases
This site, sponsored by NIH, discusses TB research, gives links to scientific publications, provides graphical explanations of TB, and archives NIAID news releases on the disease.

Stop TB Partnership
From its website: "The Stop TB Partnership was established in 2000 to eliminate tuberculosis as a public health problem and ultimately to realize a world free of TB. It comprises a network of more than 500 international organizations, countries, donors from the public and private sectors, and nongovernmental and governmental organizations that have expressed an interest in working together to achieve this goal.

World Health Organization
The WHO's Stop TB Department site includes a fact sheet, worldwide statistics on TB back to the 1990s, and the WHO's "Global Tuberculosis Control" report.