Twenty Americans who traveled to El Salvador earlier this year to rebuild a church returned with serious cases of histoplasmosis, a lung disease caused by an airborne fungus, the federal Centers for Disease Control and Prevention has reported.

More than half of the 33 church volunteers from Pennsylvania and Virginia got sick, including six who had to be hospitalized, according to the current issue of the Morbidity and Mortality Weekly Report. The volunteers traveled in separate groups but worked at the same church last January and February.

“They were a healthy group, so this was a bit surprising,” said Dr. Katie Kurkjian, an epidemic intelligence services officer with the Virginia Department of Health who wrote the report. “That suggests there was a high level of exposure.”

The fungus that causes histoplasmosis grows in soil contaminated with bird or bat droppings. The spores become airborne when the soil is disturbed.