Story highlights Idiopathic pulmonary fibrosis is a chronic, progressive lung disease with poor prognosis

Of nine patients, seven died during the reported 16-year period

(CNN) A cluster of cases of a progressive lung disease occurred among dentists and other dental workers treated at one Virginia care center, according to Thursday's Morbidity and Mortality Weekly Report from the US Centers for Disease Control and Prevention.

, is a chronic, progressive lung disease with a poor prognosis. The cause is unknown. Of nine patients, referred to as a cluster, seven died during the reported 16-year period. The disease, called idiopathic pulmonary fibrosis , is a chronic, progressive lung disease with a poor prognosis. The cause is unknown.

A cluster is a term used to define a collection of cases "grouped in place and time that are suspected to be greater than the number expected," Dr. Randall J. Nett, lead author of the study and medical officer with the US Public Health Service, explained in an email.

In this case, among 894 patients treated for idiopathic pulmonary fibrosis at the Virginia hospital, nine patients -- or 1% -- were identified as dentists or dental technicians. This number "was about 23 times higher than expected," Nett said.

All the cluster patients -- eight dentists and one dental technician -- were men, with an average age of 64. Though the reason for illness has not been identified, occupational exposures are possible, Nett and his colleagues wrote.

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