The death toll from an outbreak of Legionnaire's disease in Rzeszow, southeast Poland, has risen to five, local authorities said Thursday as they try to detect the contamination source.

The fifth victim was a 79-year-old women admitted to the hospital a few days ago.

"She was a patient with multiple long-term conditions, including cancer, and had been in the anaesthesiology and intensive care ward," the director of the Rzeszow municipal hospital, Grzegorz Materna, told state news agency PAP.

The cause of the outbreak - which has seen at least 71 people hospitalised - has not yet been determined.

Legionnaire's disease, caused by Legionella bacteria, is not contagious and cannot be spread directly from person to person, but can multiply in water and air-conditioning systems.

It causes pulmonary issues, especially for people with weak immune systems.

"The hypothesis of the municipal water supply network as the source of infection is being verified," the Polish health ministry said on X (former Twitter) on Thursday, after an overnight emergency meeting in Rzeszow.

But the test results of samples taken from the water system are not expected until Monday.

In the meantime, the authorities in Rzeszow, a city of nearly 200,000 residents, vowed to carry out additional disinfection works.

According to the local authorities, all five victims in the Rzeszow outbreak were elderly people.