Legionella are gram-negative, aerobe rod-shaped typically flagellated bacteria. Over 40 different species with over 60 serogroups are identified. The illness caused by Legionella bacteria is attributed to Legionella pneumophila serogroup 1. Infection is caused by inhalation of Legionella contaminated airborne water droplets or Legionella contaminated water by-passing the natural gag-reflexes and entering the lungs instead of the stomach.

The two most common forms of illness are:

* Legionnaires’ disease: a bacterial pneumonia. If the infection is not treated, the mortality is up to 80%. If it is treated, the mortality is approximately 20%.
* Pontiac fever: a milder, non-pneumonic, flu-like illness that does not require hospitalization.
Legionella bacteria are not completely removed during drinking water treatment (drinking water is not sterile). At water temperatures below 20 °C they can survive but not reproduce.