My 65-year-old mother has had numerous recurrent lung infections. She has been sick for three months nonstop. She will be on an antibiotic, but as soon as she finishes, her fever goes up again to 101-103. She is very fatigued and was hospitalized once. I have looked for possible causes in her apartment. There is a moldy substance on the metal fins of the central air conditioner. Could she possibly have something like Legionnaire's disease? Is there a medical test that could check out this possibility?

B.M.

Your mother has an illness that has been going on for three months. Every time she receives antibiotics, she gets better but then relapses. From an infectious-disease standpoint, such patterns tell us a great deal.

If a person has a feverish illness that does not respond at all to antibiotics, then the options are that the illness may not be caused by bacteria (because antibiotics kill bacteria but not other microbes), the bacteria may be resistant to that antibiotic, the disease may be too advanced for anything to help or the infection may be in a place where that antibiotic or any antibiotic does not reach. Probably the most common cause is the first one: The disease is not bacterial. Many physicians will treat a feverish patient with an antibiotic, and if the patient does not improve, the doctor will simply switch antibiotics. This rarely benefits the patient unless the physician actually uses laboratory testing to identify the bacterium causing the disease and changes antibiotics based on the results.

Now, your mother DID respond to antibiotics but relapsed several times. This suggests that her condition is caused by a bacterium. Molds in the house could cause reactions in the lung that are indistinguishable from pneumonia, but in such a case I would not expect her to get better after taking antibiotics. A few conditions are characterized by improvement then relapse. The first, and least likely, is that she had a simple pneumonia but was not treated long enough. I doubt this occurred. Nowadays, we tend to overtreat bacterial infections, using longer courses of antibiotics than are necessary. Also, I am certain that by now one of your mother's doctors has tried a long course of antibiotics.

A few infections improve with a course of antibiotics but are not cured without surgery or a very, very long course of treatment. A lung abscess (collection of pus), which your mother may indeed have, requires months of antibiotics for cure. Another type of abscess in the chest is called a "pleural empyema." This is an infection of the tissue outside the lung. It is protected somewhat from the body's defenses, and antibiotics cannot penetrate this region very well. Pleural empyema can also be a result of a lung infection. It will also improve but will likely not be cured by antibiotics.

Another infection that will relapse after antibiotics unless they are given for four to six weeks is osteomyelitis, infection of the bone. This can be a complication of an infection elsewhere in the body, though lung infection would be an uncommon cause. Your mother would likely have pain in the infected bones, though rarely this could be overlooked. Still another possibility is endocarditis, or infection of the tissue lining the heart. Most cases of this infection require four weeks or more of antibiotics given intravenously. There can be a good initial response to antibiotics, but the patient will usually relapse and become quite ill.

In your mother's case, I think that any of the above are possible. I would get a chest X-ray and then a CT scan of the chest if necessary, to rule out an abscess and pleural empyema. I would culture the blood while your mother is off antibiotics to exclude endocarditis.

Your mother probably does not have Legionnaire's disease, pneumonia caused by the bacterium Legionella pneumophila. The elderly and those with lung disease are at risk for this infection. But it would be quite unusual to be ill for three months, with multiple relapses. If the likelihood were higher, the diagnosis could be confirmed by testing of the sputum, blood and urine.