This tough disease takes its name from an outbreak at the Pennsylvania American Legion convention held in a hotel in Philadelphia, Pennsylvania, USA, in 1976. The disease was diagnosed for the first time after 221 people contracted the illness in Philadelphia. During this outbreak 34 people died; 29 Legionnaires and their family members and five others who had been near the hotel.

The bacterium believed to cause the illness is found in soil and grows in water, such as the air conditioning units of large hotels, office complexes and hospitals.

The American Centers for Disease Control in Atlanta, Georgia estimates that 25,000 people a year get the disease, whose pneumonia-like symptoms begin two to three days after exposure to the bacterium.