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Legionella are pathogenic Gram-negative bacteriae, ubiquitously distributed in both natural and artificial water environments. They are acid tolerant (pH 2.8–pH 8.3) as well as thermo tolerant (<20°C–66°C), and thus they can adapt to and survive in diverse aquatic settings. Legionellae are known to form biofilms in aquatic environments as a survival mechanism against adverse conditions.

From a human health perspective, the Legionella species, particularly L. pneumophila is important since it is the causative agent of Legionellosis or Legionnaires' disease in humans. According to the journal "Infection Control and Hospital Epidemiology", hospital-acquired Legionella pneumonia has a fatality rate of 28%, with its source being the water distribution system. As per the Center for Disease Control (CDC) in the United States, the disease affects between 8,000 to 18,000 individuals annually with a fatality rate ranging from 5–30%.

Legionellosis or Legionnaires' Disease

The Legionella species can cause two types of Legionellosis: Legionnaires' disease and Pontiac fever. Legionnaires' disease, also called "Legion Fever”, is a severe pneumonia. Pontiac fever is a milder respiratory illness that resembles acute influenza. Legionellosis is not contagious. Common reservoirs of the bacteria include cooling towers, air conditioning units, swimming pools, domestic hot-water systems and fountains. The primary route of transmission is via aerosols. Legionella is generally not a threat to most healthy individuals. However, it can cause serious complications in immunocompromised individuals and in the elderly.