Legionella pneumophila are gram-negative bacteria found in freshwater environments; they can survive as intracellular parasites of free-living protozoa and within biofilms in building water systems. L. pneumophila multiply at temperatures between 25 and 42°C, with an optimal growth temperature of 35°C. There are 16 serogroups of L. pneumophila. In Europe, approximately 70% of Legionella infections are caused by L. pneumophila serogroup 1 and 20–30% by other serogroups. These bacteria cause respiratory disease (legionellosis) in humans when a susceptible host inhales aerosolized water containing the bacteria or aspirates water containing the bacteria. Legionellosis classically presents as two distinct clinical entities: Legionnaires’ disease, a severe multisystem disease involving pneumonia, and Pontiac fever, a self-limited flu-like illness. Urine antigen tests and cultures of sputum or bronchoalveolar lavage are the most suitable clinical laboratory tests for Legionella. The new macrolide antibiotics, such as clarithromycin and azithromycin, show more effective in-vitro activity and a better intracellular and tissue penetration than erythromycin, as do the quinolones. Hospitals represent ideal locations for Legionnaires’ disease transmission: at-risk individuals are present in large numbers; plumbing systems are frequently old and complex, favouring multiplication of the organism; and water temperatures are often reduced to prevent scalding of patients. Safe water is vital to ensure patient safety and reduce costs where waterborne infections cause increasing morbidity, mortality, treatment costs, longer hospital stays and compensation claims. The WHO has developed Water Safety Plans for preventing or controlling the risks through system assessment, monitoring, surveillance and management/communication, so that health outcomes can be improved, that is a systematic approach is required to secure microbial safety.