LISBON (Reuters) - The death toll from an outbreak of Legionnaires' disease in a large Lisbon hospital doubled to four on Friday, Portugal's health authority DGS said, but the number of new cases being discovered appeared to subside.

The severe form of pneumonia has made another 40 people sick since it was detected on Oct. 31 at the São Francisco Xavier hospital in Lisbon's western Restelo area and six were in intensive care.

The peak of the outbreak was last Saturday when 13 new cases were discovered, while in the past few days new cases amounted to 3-4 a day and so far just one on Friday, DGS data showed.

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Most of the 44 confirmed cases involved elderly people with other chronic conditions - the disease's most common victims.

The prosecutor's office has launched an investigation of the outbreak, which provoked a bizarre incident on Wednesday when police interrupted the wake for one of the victims and took the body for an autopsy.

The authorities said experts were still working to establish the source of the bacteria.

In 2014, Portugal's worst ever outbreak - also one of the world's largest - of Legionnaires' disease killed 10 people and infected a total of 336 people. It was traced back to industrial cooling towers at the Vila Franca de Xira area just northeast of Lisbon. Most of those infected lived or worked in the area.

The disease is contracted by breathing in a mist or vapor contaminated with the Legionella bacteria, which can grow in cooling towers, showers, spas, hot tubs and other water sources. It is not transmitted directly from person to person.

The illness is named after a 1976 outbreak at an American Legion convention in Philadelphia, which killed 34 people.

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(Reporting By Andrei Khalip, Editing by William Maclean)