Legionella bacteria live in water or wet soil, and they must be inhaled to cause infection. In nature, there is nothing that will spread tiny drops of Legionella-infected water in the air. However, the modern cooling towers or evaporative condensers in large buildings make it easier for these waterborne bacteria to get in the air.
Such a tower works by spraying water through circulating air. The smaller the drops, the more efficient the cooling, so various devices are used to break large drops into smaller and smaller ones. These towers may be contaminated with limescale and algae, both of which encourage the growth of Legionella.
In older, badly designed or poorly maintained buildings, evaporated water or tiny droplets can get into air conditioning through breaches in the ducts, especially if these are too close to the cooling tower. Contaminated droplets and vapour also exit directly from the top of the cooler, potentially contaminating other towers in nearby buildings. Most cases of Legionnaires' disease appear in large institutional buildings, especially in hotels and hospitals or on cruise ships. It is transmitted via air-conditioning, heating, and cooling systems.
Legionella can easily survive in hot water tanks. It has been found coming out of shower heads that create spray fine enough to reach the lungs. Electric humidifiers and hospital assisted-breathing equipment have also been responsible for cases of Legionnaires' disease. Occasionally, it's been transmitted outdoors, usually where heavy digging disturbs wet soil.
The bacteria may also be transmitted by aspiration. Aspiration occurs when liquids or food is accidentally inhaled into the lungs instead of being swallowed and going into the stomach.
While anyone can get Legionnaires' disease, some people are more susceptible than others. Middle-aged people and seniors are more at risk. The following conditions increase the risk of Legionnaires' disease:
- chronic kidney disease
- corticosteroid use (e.g., prednisone, dexamethasone)
- organ transplant
- anything else that weakens the immune system, such as a viral infection
Legionnaires' disease doesn't spread from one person to another, so it's not contagious. Evidence also suggests that people can't get it from drinking contaminated water unless it is aspirated and breathed in.