DARMSTADT, Germany -- At least seven old people have died in an old people's home in Germany in less than a week due to a mysterious virus, local authorities said Monday.

The first death occurred Wednesday of last week in the retirement home at Darmstadt in western Germany.

A further 17 of the 152 residents had been hospitalized with high temperatures and breathing difficulties, authorities said.

Regional health authorities have submitted samples of drinking water from the premises to laboratory tests to establish whether legionnaire's disease could be the cause, AFP reported.

Test results were expected Tuesday.

Legionnaire’s is an infectious disease causing a severe form of pneumonia. It is produced by a bacterium that colonizes in warm, stagnant water and may be found in such places as air cooling systems, humidifiers and water taps.

The bacterium was named Legionella pneumonia when, in 1976, 29 people died after suddenly becoming ill at an American Legion convention in Philadelphia.

Tests are also being made in Darmstadt to find out whether the series of deaths might be linked to an intense heat wave attacking Europe this summer.

A Berlin research establishment, the Robert Koch Institute, which specializes in study of contagious diseases, confirmed an increase during August of the number of deaths in retirement homes in the German state of Hesse, in which Darmstadt is located, as well as in the adjacent states of North Rhine-Westphalia and Baden-Wuerttemberg.