Legionella: from environmental habitats to human disease

Water is the main reservoir for legionellae, and these bacteria are found in freshwater environments worldwide. Legionellae survive in aquatic and moist soil environments, in biofilms, as intracellular parasites of free-living protozoa. Protozoa naturally present in the environment support the intracellular growth of legionellae in vitro. Much of our understanding of the pathogenesis of legionellosis is based on studies of the infective process in protozoa and human cells.