The article provides a comprehensive overview of Legionnaires' disease, including its clinical presentation, microbial ecology, pathogenesis, diagnosis, and epidemiological trends. Legionnaires' disease is a severe form of pneumonia caused by Legionella bacteria, which are found in freshwater environments and can be transmitted to humans through aerosolized water. The bacteria are intracellular parasites of freshwater protozoa and can cause respiratory disease when inhaled or aspirated. The disease can present as Legionnaires' disease, a severe multisystem illness, or Pontiac fever, a self-limited flu-like illness. The primary species causing legionellosis is *Legionella pneumophila*, which accounts for about 90% of reported cases in the United States. The bacteria are detected through various methods, including culture, direct fluorescent antibody (DFA) staining, serologic testing, and urine antigen detection. The article also discusses the importance of biofilms in the survival and multiplication of Legionella in building water systems and the role of protozoa in the intracellular life cycle of the bacteria. The diagnosis of Legionnaires' disease is challenging due to the lack of clinical awareness and the fastidious nature of the bacteria, but recent advancements in molecular techniques have improved diagnostic capabilities. The article concludes with a discussion on the prevention and control of legionellosis, emphasizing the importance of maintaining building water systems to prevent outbreaks.The article provides a comprehensive overview of Legionnaires' disease, including its clinical presentation, microbial ecology, pathogenesis, diagnosis, and epidemiological trends. Legionnaires' disease is a severe form of pneumonia caused by Legionella bacteria, which are found in freshwater environments and can be transmitted to humans through aerosolized water. The bacteria are intracellular parasites of freshwater protozoa and can cause respiratory disease when inhaled or aspirated. The disease can present as Legionnaires' disease, a severe multisystem illness, or Pontiac fever, a self-limited flu-like illness. The primary species causing legionellosis is *Legionella pneumophila*, which accounts for about 90% of reported cases in the United States. The bacteria are detected through various methods, including culture, direct fluorescent antibody (DFA) staining, serologic testing, and urine antigen detection. The article also discusses the importance of biofilms in the survival and multiplication of Legionella in building water systems and the role of protozoa in the intracellular life cycle of the bacteria. The diagnosis of Legionnaires' disease is challenging due to the lack of clinical awareness and the fastidious nature of the bacteria, but recent advancements in molecular techniques have improved diagnostic capabilities. The article concludes with a discussion on the prevention and control of legionellosis, emphasizing the importance of maintaining building water systems to prevent outbreaks.