Tuberculosis

Causes

Mycobacterium tuberculosis, the bacterium that causes tuberculosis,spreads in microscopic droplets that are released into the air when someone with the untreated, active form of the disease coughs, speaks, laughs, sings or sneezes.

Although tuberculosis is contagious, it's not especially easy to catch. In general, you need long-term contact with an infected person to become infected yourself. You're much more likely to contract tuberculosis from a family member or close co-worker than from a stranger on a bus or in a restaurant. A person with nonresistant active TB who's been effectively treated for at least two weeks is generally no longer contagious. Rarely, a pregnant woman with an active TB disease may pass the bacteria to her fetus.

TB infection versus active TB
Although TB can affect other organs and tissues, it primarily attacks your lungs. Approximately two to eight weeks after your lungs are infected with M. tuberculosis, your immune system springs into action. Macrophages — specialized white blood cells that ingest harmful organisms — begin to surround and "wall off" the tuberculosis bacteria in your lungs, much like a scab forming over a wound. If the macrophages are successful, the bacteria may remain within these walls for years — alive, but in a dormant state. In this case, you're considered to have TB infection and you'll test positive on the TB skin test, but you won't feel sick or have symptoms and you can't transmit the disease to others.

But sometimes your immune defenses fail, even if you're otherwise healthy and don't have a compromised immune system. In that case, TB bacteria actually begin to exploit macrophages for their own survival, causing the white blood cells to form into tightly packed groups called granulomas. The bacteria multiply inside the granulomas, which eventually may enlarge into noncancerous tumor-like nodules. The centers of these nodules have the consistency of soft, crumbly cheese.

Over time, the centers can liquefy and break through the granulomatous wall surrounding them, spilling bacteria into your lungs' airways and causing large air spaces (cavities) to form (active TB). Filled with oxygen, the air spaces make an ideal breeding ground for the bacteria, which multiply in enormous numbers. The bacteria may then spread from the cavities to the rest of your lungs as well as to other parts of your body.

Active TB is contagious and serious
If you have active TB, you're likely to feel sick, although it's possible to have an active infection in your lungs without having symptoms. But even if you don't feel sick, if the disease is active you still can transmit it to others by coughing, sneezing or talking.

Without treatment, many of the people with active TB die. Those who survive develop chronic, debilitating symptoms, such as chest pain and a cough with bloody sputum, or their immune system recovers and the disease goes into remission.

Sometimes active TB can develop years after the initial infection. This occurs when your immune system can't keep dormant TB bacteria at bay, and the walled-off germs become active. A number of factors can weaken your immune system, including aging, drug or alcohol abuse, malnutrition, chemotherapy, prolonged use of prescription medications such as corticosteroids, and diseases such as HIV/AIDS. About one in 10 people who have TB infection goes on to develop active TB sometime inhis or her life. The risk is greatest in the first year after infection, but the disease may not resurface for decades.

Why is TB on the rise?
In the United States, cases of tuberculosis began declining steadily in the 1940s and 1950s mainly because of antibiotic therapy and improved public health programs. Yet the disease is still a serious health problem. Millions of Americans are infected with TB without having symptoms, and some of them will go on to develop active TB.

The situation in other countries is far worse. Overall, about one-third of the world's population is infected with tuberculosis. New infections and deaths from the disease are increasing. Hardest hit are sub-Saharan Africa and Southeast Asia. A number of factors have contributed to the global TB crisis, but the leading cause is the spread of HIV, the virus that causes AIDS. Tuberculosis and HIV have a deadly relationship — each fuels the progress of the other.

Infection with HIV suppresses the immune system, making it difficult for the body to control TB bacteria. As a result, people with HIV are many times more likely to progress from dormant to active disease than are people who aren't HIV-positive.

TB is one of the leading causes of death among people living with AIDS — not only because they're more susceptible to TB, but also because TB can increase the rate at which the AIDS virus replicates. One of the first indications of HIV infection may be the sudden onset of TB — often in a site outside the lungs (extrapulmonary TB).

Other factors contributing to the spread of TB in the United States and elsewhere include:

• Crowded living conditions. TB spreads most easily in cramped, crowded, poorly ventilated spaces. Incidence rates in prisons, juvenile detention centers and homeless shelters are higher than that in the general population. TB bacteria also can flourish in nursing homes because older adults often have immune systems weakened by illness or aging.
• Increased numbers of foreign-born nationals. Although TB rates for people born in the United States are declining, the incidence among people from other parts of the world, especially Africa, Asia and Latin America, is increasing. More than half of the reported TB cases in the United States are in people born outside the country.
• Increased poverty and lack of access to medical care. The world's poor, in America and in other countries, are more likely to have TB but the least likely to receive medical care. The problem is compounded because people living in poverty and in unstable political situations often move or migrate and therefore may not complete their treatment, leading to drug-resistant forms of the disease.

• Increase in drug-resistant strains of TB. For each major TB medication, there's a TB strain that resists its treatment. Even more dangerous are strains that are resistant to at least two anti-TB drugs, leading to a condition called multidrug-resistant TB (MDR-TB). People with untreated MDR-TB are highly contagious and can transmit this serious type of TB to others.

  Although MDR-TB can be successfully treated, it's much harder to combat than regular TB and requires long-term therapy — up to two years — with drugs that can cause serious side effects. MDR-TB bacteria can develop when people don't complete their entire course of medication or fail to take their medications as prescribed, when health care professionals prescribe the wrong kinds of treatment, or when the drug supply is inconsistent — a particular problem in impoverished or war-torn nations.