The Legionella organism is a Gram negative, rod
shaped bacterium that can cause pneumonia
(Legionnaires’ disease) or a flu-like illness (Pontiac fever). 
It was first identified and recognized as causing disease during the outbreak that occurred in conjunction with the American Legion Convention in Philadelphia in 1976. There are over 50 species of Legionella and 18 of those can cause disease.

It is thought that the species Legionella pneumophila causes most of the
infections. Legionella is a fastidious organism, meaning it has specific growth requirements that need to be met in order for it to survive and grow.

 

Some of these growth requirements are: Temperature above 68° F ; Iron ; L-Cysteine and Biofilm (particularly protozoans) Unlike some other bacteria, it can survive at lower temperatures (even below freezing), at lower dissolved oxygen levels and is somewhat resistant to chlorine disinfection. Certain plastics and organics can provide nutrients for growth. These attributes make our modern day plumbing systems a good habitat for the organism.