Philadelphia and its surrounding counties have been identified as a cluster area for a rare lung disease. It's one of seven such clusters a recent National Institutes of Health study found in the U.S.

The disease is caused by Mycobacterium avium complex, often known by its acronym MAC, and was first dubbed "Lady Windermere syndrome" after a character in an Oscar Wilde play because of those it usually strikes.

"The profile re-emerges time after time. Fifty-five to 75- or 80-year-old women, otherwise generally healthy, Caucasian, and not overweight," said Dr. John Hansen-Flaschen, chief of pulmonary allergy and critical care at the University of Pennsylvania. Usually, patients are also of higher than average socioeconomic status.

He said MAC infections can cause a persistent cough that, if left untreated by a long-term course of antibiotics, can become severe and progress to fever, shortness of breath, cavities in the lungs and, in rare cases, death.

"For two decades now, pulmonary physicians in the Philadelphia area have been encountering this on a very regular basis, much more so, it seemed to us, than pulmonary physicians in other regions of the country," Hansen-Flaschen said. "We long suspected that the Philadelphia area might be a hot spot."

The study found reported rates of infection among Medicare patients twice as high here as the national average. However, Stephen Ostroff, director of the bureau of epidemiology at the Pennsylvania Department of Health, warned the study is too small to confirm there is really a disproportionate problem here.

"The total number of patients that are reported in this study with the lung infection is 2,548 over an 11-year time period, which translates to about 230 or so cases for the entire country per year," Ostroff said. "I'm not sure that you can draw many conclusions based on that small a number of cases."

He said doctors in southeastern Pennsylvania could be more aware of the diagnosis, or patients could be more active in seeking treatment, or more likely to have pre-existing conditions.

The bacteria that causes the disease are ubiquitous in soil and drinking water throughout the country. Doctors don't know why it only sickens some people.