WARSAW, Poland (AP) — Just one new case of Legionnaires’ disease was reported Tuesday in southeast Poland in an area close to the Ukraine border, an indication the spread of the disease has been curbed following chlorine disinfection of the water system.

The latest fatality was registered on Friday, bringing to 19 the death toll since the outbreak in July. A total of 166 people have been infected in the region of Rzeszow, some 80 kilometers (50 miles) from the border.

It's a key transit hub for international military support for Ukraine following Russia’s full-scale invasion last year. Some 10,000 U.S. troops are stationed in the area.

Health authorities, prosecutors and internal security agency were still looking for the source of the pulmonary infections in a strategically sensitive region. Experts say it could come from warm water in rarely used plumbing amid hot weather.

Fatalities from the disease were among elderly people who also suffered from other health issues like cancer, authorities said.

Legionnaires’ disease is a lung infection that comes from inhaling infected water spray. It’s not spread through drinking water.

It is usually caught in places like hotels, hospitals or offices where the bacteria have got into the water supply, for example in air conditioning systems or taps and showers that are not in frequent use.

The germs multiply in temperatures of between 20 and 50 degrees Celsius (68-122 degrees Fahrenheit). The disease is treated with antibiotics.