Legionellosis is a bacterial disease of environmental origin that it usually presents two distinct clinical forms: Lung infection or «Legionnaires Disease», characterized by pneumonia with high fever, and not pneumonic form, known as "Pontiac fever", which manifests itself as an acute febrile syndrome and slight injured. The Legionella infection can be acquired in the community and the hospital area. In both cases the disease can be associated with several types of equipment and buildings. It can occur in the form of isolated or sporadic cases.

Legionella is an environmental bacterium that can survive in a wide range of physical and chemical conditions, multiplying between 20 ° C and 45 ° C, being destroyed at 70 ° c. Its optimal growth temperature is 35-37 ° C. Their natural ecological niche are surface waters, such as lakes, rivers, ponds, forming part of their bacterial flora. From these natural reserve, bacteria can colonize the systems of cities and through the network of water distribution, is incorporated into water (hot or cold) systems or other systems requiring water for their operation such as cooling towers.

In these installations, poorly designed, without maintenance or with an inadequate maintenance, it sometimes favors the water stagnation and accumulation of nutrients bacteria products, such as sludge, organic matter, corrosion matters and amoebas, forming a biofilm. The presence of this biofilm, along with a favorable temperature, explains the multiplication of Legionella to infective concentrations for human beings. If there is an aerosol producing mechanism in the installation, bacteria can disperse into the air. Water drops containing the bacterium can remain suspended in the air and penetrate in the respiratory system by inhalation.

The facilities that most often are contaminated with Legionella and have been identified as sources of infection are the systems of distribution of hot and cold sanitary water and the evaporative water cooling equipment, such as cooling towers and evaporative condensers, both in hospitals and in hotels or other type of buildings.