Legionnaire's disease is caused by a bacteria called Legionella. It got it's name in 1976 when a group of conventioneers were attending a function at a American Legion in Philadelphia. The organism causes a severe type of pneumonia (inflammation of the lungs).
Signs and symptoms of this type of pneumonia are similar to other bacterial infections and consist of cough, headache, fever, chills, body aches, and shortness of breath or dyspnea. The pathogenic bacteria are found everywhere but typically in warm water like in hot tubs. It may inhabit ventilation ducts in buildings. It is hypothesized that the conventioneer's in Philadelphia contracted it in this way. It is contracted by inhaling the bacteria.
I have no information that indicates it is more common in war zones. Smokers or people who are immunocompromised are more susceptible than others.
Legionnaires' disease is the most severe of two forms an infection of legionellosis that manifests as pneumonia. The less severe pontiac fever is caused by the same infection; however, it produces a condition much more resemblant of influenza.
The infection usually occurs after inhaling an aerosol that has the bacteria, which originates from any infected water source. When the infected water is disturbed the bacteria is evaporated into the air that are then inhaled. Areas with poor ventilation like prisons are the most vulnerable to spreading the infection because it spreads through the entire room. The disease is mostly associated with hotels, cruise shops, hospitals with old ventilation systems because of the number of people in a smaller space in all of them.
It is not isolated just to war zones, the name is an historical one dating to the first outbreak in Philadelphia in 1976 in the Bellevue Stratford Hotel where members of the American Legion had gathered for the American Bicentennial. Outbreaks since then have been in the UK in 1985, the Netherlands in 1999, Australia in 2000, Spain in 2001, the UK again in 2002, and Norway in 2005.