The Legionnaires' Disease (LD) Bacillus was first recognized following a large outbreak of pneumonia at a convention of American military veterans in 1976 (1). In the context of other microbial infections, this is a relatively recent event. Since then, considerable progress has been made in the understanding of legionellae and the epidemiology of the infections they cause. More than 40 species of Legionella have been identified and described, and these organisms have been found to be widely distributed in nature. They occur in almost all natural bodies of fresh water (2) (although they may be present at only low numbers) and in most man-made aquatic systems (3). It appears that, in then natural environment, legionellae grow in association with other microorganisms and colonize surfaces at the aqueous/solid interface. Here, together with their protozoa1 hosts, other bacteria, and algae they form a complex consortium of organisms loosely termed "biofilm."