Legionella species are consistently recognized as one of the most common causes of pneumonia. <em>Legionella pneumophila</em> serogroup 1 is responsible for most human infections and is the causative agent of Legionnaires' disease, also known as <em>legionellosis</em>. The bacteria occur naturally in freshwater however, artificial water systems, such as large building air-conditioning systems, provide environments conducive to growth and can cause a disease outbreak.