The legionella bacterium grows at a slower rate than most other fresh water bacteria. Legionella depends upon the presence of appropriate nutrients and minerals for growth. Two of the most important are an amino acid called L-Cysteine and iron. Laboratory experiments have shown that some species of legionella can multiply intracellularly within certain free-living protozoa. In addition to protozoa, algae and non-legionella bacterial have been shown to play important roles in the proliferation of legionella.
Algae provide growth stimulation of legionella through substances produces by the algae.
Of the two forms the pneumonia is much more serious. The pneumonia form of legionellosis is transmitted by the breathing in the water vapors containing legionella. This allows the bacterium to infect the lung tissue. The incubation period can be anywhere from 2 to 10 days long. This form of legionellosis is generally thought to be seasonal with the greatest number of outbreaks occurring in the summer and fall.