Although it is possible to contract the illness from Legionella growing in home plumbing systems, most cases have been traced to plumbing systems and cooling towers in large buildings. This may be because larger piping networks are generally more conducive to Legionella growth. Also, the air conditioning systems for large buildings often include cooling towers, which contain a pool of warm water in which Legionella can flourish. Whirlpool spas are also a major source of Legionnaires' disease.