Health officials are investigating two cases of the potentially serious type of pneumonia in patients treated at University of Washington Medical Center cardiac units, officials said Thursday.

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Seattle-area health officials are investigating two cases of Legionnaires’ disease, a potentially serious type of pneumonia, in patients treated in the cardiac units at the University of Washington Medical Center (UWMC).

One case was reported Aug. 26 in a patient who may have been exposed to Legionella bacteria either at the hospital or in the community, officials with Public Health – Seattle & King County said Thursday. A second case was reported Tuesday in a person who was at UWMC during the entire exposure period.

One patient remains hospitalized and is being treated for the infection. The other has been released.

Legionella is a type of bacteria found in nature, typically in places like freshwater lakes and streams. However, it can grow within man-made water systems, including air-conditioning units, plumbing systems and hot-water tanks and heaters.

Infection typically occurs when people breath in mist or vapor contaminated with the bacteria. It causes symptoms including cough, shortness of breath, fever, muscle aches and headaches and can be dangerous in people with weak immune systems.

No source of the UW cases has been detected; an investigation is under way, Seattle health officials said. UWMC has hired a specialist to examine the hospital’s heating, ventilation and air-conditioning systems, said Meredith Li-Vollmer, a spokeswoman for public health.

Hotel air-conditioning units, whirlpools and ice machines have been linked to the spread of the disease, which in the U.S. sends an estimated 8,000 to 18,000 people a year to the hospital, according to the Centers for Disease Control and Prevention. In Washington state, the number of cases varies each year from fewer than 10 to more than 50. In 2014, 63 cases of Legionnaires’ disease were reported, including eight deaths, according to the state Health Department.

The disease was first identified after an outbreak in 1976 sickened many attendees at an American Legion meeting in Philadelphia.