Legionellaceae is a family
of disease-causing Gram negative aquatic aerobic flagella
ted rod-shaped bacteria
. Diseases caused by members of this family includes legionellosis, more commonly known as Legionnaire's disease
Legionellosis is caused by Legionella pneumonophila; it begins with flu-like symptoms, then moves on to high fever, shaking chills, headaches, diarrhea, pneumonia, and pleurisy, and can result in death. The disease is highly contagious.
The bacteria which causes this disease is only harmful when tiny droplets of water floating in the air containing the bacteria are inhaled, and does not cause harm when it is present in drinking water.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.