The risk of contracting Legionnaires' Disease from residential plumbing is rare, but some inexpensive measures can be taken.
Although large piping networks are more conducive to the growth of Legionella bacteria (legionellae) than smaller systems, plumbing in houses and apartments still can harbor the bacteria, and have indeed been implicated in cases of Legionnaires' disease.
Biofilm is a primary factor associated with legionellae in plumbing systems. Biofilm is a slimy coating that is formed as microbes attach to underwater surfaces (e.g., the inside of a pipe). Biofilm and scale that form in valves and fittings and on pipe walls not only feed legionellae, but also protect it from hot water and chlorine. Fragments of the outer layer of biofilm can flake off and flow into the water, releasing potentially high levels of legionellae and other microbes into the system.