My 62-year-old mother was diagnosed last year with tuberculosis. They said there was no medicine she could take. She does not smoke but has worked as a waitress for 30 years. Two years ago she took care of her mother-in-law who was dying of cancer. Some people say you can get TB by being with someone who is terminally ill. Is that true? She has been sick with bronchitis every few months, and this week they found a dark spot in her right lung. If she does have tuberculosis, is there some treatment for her at her age?

L.C.

One of my favorite topics to discuss is tuberculosis, or TB. Your question is complex, so let me try to break it down.

First, tuberculosis is caused by a bacterium called Mycobacterium tuberculosis, or MTB for short. The microbe is transmitted through the air and inhaled. Actually, it is not as contagious as some people think -- you usually only get infected when you are exposed to someone for an extended period, not just a minute or two. There is also an important difference between having a positive TB skin test, which means you have been infected with MTB, and having the disease called tuberculosis, which occurs only if your own immune system can't control the MTB infection.

Infection occurs when a person inhales the MTB into his or her lungs. In some countries, and in the United States many years ago, infection with MTB was very common. Currently more than one-third of all people in the world are affected.

Most people do not have any symptoms at the time of infection. Some will feel like they have the flu, and a few will immediately develop a severe pneumonia. However, in most people the immune system will control the infection, so that there are no symptoms or just a short illness that goes away by itself. At this time, or shortly thereafter, if the person is given a TB skin test, it will be positive. (The TB test involves injecting into the skin a small amount of bacterial protein called the purified protein derivative, or PPD. A positive reaction causes swelling at the site of the injection.)

Later in life, especially when a person is old or if a weakness develops in the immune system, the bacteria may reactivate and cause the disease we call typical tuberculosis. The symptoms include cough, fever, weight loss and night sweats. Most people with these tuberculosis symptoms were actually infected years earlier.

A person who has a positive TB skin test but none of the symptoms of tuberculosis can take medicine to decrease the chances of ever coming down with the disease. The antibiotic called isoniazid (INH) is often prescribed for six to nine months, and that decreases the risk of later developing tuberculosis by about 75 percent. INH treatment is recommended for everyone with a positive TB skin test who is under 35 years old, because the drug is safest in the young. In older patients, INH is recommended only in certain circumstances. Perhaps this is why your mother was not offered any treatment.

Now, does the spot in your mother's lung mean she has tuberculosis? It's possible. But before she is treated, we would have to know for certain that the spot was indeed caused by MTB, and not by another infection, a tumor or something completely different. First, her doctor would probably examine your mother's sputum (phlegm) under the microscope, and also send a sample to a lab to grow any bacteria present. If bacteria are found, these would be tested to see if they are MTB. If not, she will probably need a lung biopsy to look for MTB and to rule out lung cancer.

How do we treat active tuberculosis? The disease requires a combination of three or four drugs, some of which are taken for six to nine months. In general, if the patient is not too sick to start with, he or she can usually be successfully treated, regardless of age.

One last note: you cannot get TB from someone who is terminally ill, unless of course they have tuberculosis.