A total of 175 people are now hospitalized in a Urals region following a pneumonia outbreak believed to have been caused by contamination in the hot water supply, local authorities said.

The infection spread in the Sverdlovsk Region after annual maintenance to hot water pipes supplying homes. Four people are confirmed to have died of pneumonia. A local Health Ministry spokesman said one of the patients is in intensive care.

Of the patients, 150 have been diagnosed with Legionnaires' disease, a severe form of pneumonia, and in 66 of these cases the diagnosis has been confirmed by laboratory tests.

The spokesman said 31 people had already been discharged from hospital and that most patients were recovering following therapy.

According to the ministry's data, one other patient admitted in the outbreak has died, but a pneumonia diagnosis has not been confirmed.

The outbreak of the infection was registered on July 19 in the town of Verkhnyaya Pyshma, when 35 people were put in hospital. All the patents are adult locals. The infection is believed to have spread through the hot water supply after annual maintenance work. An investigation is underway.

Legionnaires' disease, a type of lung infection, can be contracted by breathing steam from water carrying the bacteria. Infection is often spread from hot baths, showers or air conditioning. The bacteria do not spread from person to person.

Most apartment blocks in Russia receive hot water via mains supply, directly from thermal plants, rather than from water heaters installed within buildings. The supply is shut down for a few weeks each summer for maintenance work.