Although one does not find the origin of the contamination in the human half of the cases of legionellosis, one knows that this disease is the consequence of the almost obligatory contamination of the networks of installations of hot water by Legionella pneumophila, and the inhalation by the man of infected droplets. Pathology generally consists of a relatively serious pneumopathy. The control of the level of contamination of the various producing hydrous installations of aerosols is imperative to avoid the serious medical consequences, which cannot be prevented by an action on the human target. The majority of the currently identified tanks are the air and cool towers and the distribution networks of hot water. The taking into account of this risk in the hospitals or thermal led to the implementation of many measurements of disinfection and control, which start to show a certain effectiveness on which has occurred of new cases in these establishments, today in clear reduction.