The case report, published in The Lancet, describes an unnamed 82-year-old woman in Rome who was hospitalised with fever and breathing problems in February 2011.
Swiftly diagnosed with infection by the Legionella pneumophila germ, she died two days later of septic shock despite being given heavy doses of antibiotics.
During the two- to 10-day time it would have taken for the bacteria to incubate, the patient had only left her house twice, both times to attend appointments at the dentist.
Samples of water were taken from the dentist's tap, from the waterline -- the tube that supplies water to tooth scalers and handpieces used by the dentist -- and from the high-pressure pump supplying the waterline itself.
All three sources tested positive for L. pneumophila, but especially in water taken from the pump.
Genetic sequencing found that the germs there matched the bacteria which killed the patient. The bug turned out to be a particularly virulent sub-strain called Benidorm.
After cleaning with hydrogen peroxide solution and bleach, the water unit was free of contamination.
The case is unusual, as outbreaks of Legionnaire's disease are generally caused by air-conditioning systems, hot-water systems, spas and fountains that are not properly cleaned or maintained.
Warm temperatures and periods of water immobility provide a breeding ground for the bacteria. Distributed in fine droplets by a spray, the bacteria are then breathed in. Elderly people or individuals with poor immune systems are those most at risk.
Previous research has shown that dental waterlines can be contaminated by the germ, but this is the first known case where illness has occurred.
"As far as we are aware, no case of Legionnaire's disease has been associated with this source of infection," says the report, headed by Maria Luisa Ricci at the Istituta Superiore de Sanita in Rome.
"The case here shows that the disease can be acquired from a dental unit waterline during routine dental treatment. Aerosolised water from high-speed turbine instruments was most likely the source of the infection."
The case report puts down a series of recommendations, including use of filters, continuous circulation of disinfected water and using sterile water instead of tap water.