MILWAUKEE -

Health officials are investigating an outbreak of Legionnaires' disease that has sickened 20 people in Milwaukee County.

Fourteen cases involve adults in Milwaukee with underlying health conditions.

No deaths have been reported. But a Milwaukee Health Department official tells the Journal Sentinel that four of the Milwaukee patients are still hospitalized. Their conditions were unavailable.

The disease takes its name from an outbreak at a Philadelphia convention of the American Legion in 1976. Legionnaires' disease is a severe form of pneumonia caused by a bacterium known as legionella. The disease does not spread person to person. People get sick by inhaling the bacterium.

The first case of Legionnaires' in Milwaukee was reported June 1, but a cluster of cases didn't become apparent until after the Fourth of July weekend.