Whether your building is newly constructed or even 100+ years old, chances are, sooner or later, it can become colonized with opportunistic premise plumbing pathogens (OPPP’s) such as Legionella.
As facultative intracellular pathogens, meaning they are capable of living either inside or outside of other host cells, Legionella enters your building water system(s) as an endosymbiont of another organism. Translated: they hitch a ride into your building inside of amoeboid cells which, incidentally, function as their natural host organisms in the aquatic environment.