WARSAW, Poland (AP) — The death toll from Legionnaires’ disease in Poland has risen to 16 with another 140 people infected in the southeastern region close to the border with Ukraine, health authorities said Wednesday.

The region of Rzeszow, some 80 kilometers (50 miles) from the border, is 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.

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

Laboratory tests have confirmed the presence of Legionnaires' disease germs in the city’s water pipeline system. Authorities are still looking for the source of the infections, the numbers of which were unprecedented in the region. The Internal Security Agency was also checking for any signs of malicious action.

Experts say it could come from rarely used plumbing, where the germs could have spread in high temperatures during the recent heat wave.

Chlorine was applied to Rzeszow’s water system over the weekend for disinfection.

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.