An outbreak of Legionnaires' disease in the southern Polish city of Rzeszow has killed five people so far, a doctor said on Thursday, as authorities scrambled to find the source of the bacterial infection, which they believed was in the water supply.

The most common form of transmission is inhalation of contaminated aerosols produced in conjunction with water sprays, jets or mists of contaminated water sources, according to the World Health Organisation (WHO).

The fifth and latest victim in Rzeszow was a 79-year-old woman, Grzegorz Materna, the director of Rzeszow's City Hospital told state-run news agency PAP.

"She was a patient with multiple diseases, including cancer... Tests confirmed the presence of bacteria," PAP quoted Materna as saying.

5 PEOPLE WHO STAYED AT NEW HAMPSHIRE CAMPGROUND INFECTED WITH LEGIONNAIRE'S DISEASE

Legionnaires' disease, caused by the legionella bacteria, can result in a severe form of pneumonia.

The bacteria has been found in 76 patients hospitalised in Rzeszow and the surrounding area, PAP reported on Thursday.

Rzeszow mayor Konrad Fijolek said that authorities were testing water samples for legionella and that the results would be known on Monday.

Meanwhile, the city was increasing the levels of chlorine, UV rays and ozone used for water treatment to the maximum. The water system will also be rinsed and disinfected, Fijolek said.

The health ministry said early on Thursday that a crisis team under the leadership of health minister Katarzyna Sojka had been formed to look into the outbreak.