The legionnella bacterium is actually present in water but requires a warm temperature to become active. In order to become infected the water has to be heated to the range in which legionnella is active. It must also be in an aerosol form so that the legionnella is inhaled rather than ingested.
As well as the systems mentioned by kessenich a major contributor to airborn legionnella is water towers used to cool down water from refrigeration systems. Steps are being taken to limit exposure in this manner by doping the water with biocide or by bubbling ozone through the water before cooling.
A legionnella infection (legionellosis) can take two forms, the milder illness Pontiac fever, and the more serious Legionnaires' Disease.
The Centre for Disease Control lists the symptoms of legionnellosis as being fever, chills, dry or wet cough, muscle aches, headache, tiredness, loss of appetite, and, occasionally, diarrhea. Possibility of kidneys not functioning properly. Pneumonia. All of these are symptoms of Legionnaires' Disease
Pontiac fever symptoms are reduced to fever and muscle aches
The incubation period for Legionnaires' disease is 2 to 10 days and few hours to 2 days for Pontiac fever.
Legionnaires' Disease is gnerally treated with an antibiotic.
For more detailed information try the CDC web page on Legionnaires' Disease at http://www.cdc.gov/ncidod/dbmd/diseaseinfo/legionellosis_g.htm