It’s a lung infection, similar to tuberculosis, that is caused by a bacteria found mainly in water but also in soil and in the air. Doctors say the number of patients is growing.

The unlikely victims are white, thin, post-menopausal women who live in affluent neighborhoods around Philadelphia’s Main Line.

A study from the American Thoracic Society and the National Institutes Of Health identified 7 significant clusters of NTM around the country. One of the clusters was found in the greater Philadelphia metropolitan area where significant clusters were identified in Montgomery County.

Doctors say African-Americans and people who are heavy tend to not get NTM even though the bacteria that causes it is everywhere.

So why do only certain people develop the infection? “We don’t know,” says Dr. Leah Lande, a pulmonologist at Lankenau Medical Center. “A lot of people are trying to figure that out now but that’s not really understood right now.”

Below is a video from a local news report about this mysterious disease.