Legionella pneumophila can be found outside in soil, rivers, ponds, and lakes, but rarely cause an infection. The danger is mainly spread in the air through the building cooling tower plume that would contain Legionella. It can also be spread through decorative fountains, pools, hot tubs, whirlpools, hot water plumbing systems, hot water tanks, showers, faucets, emergency eyewash stations, etc. Most outbreaks are linked to large buildings where the bacteria can easily grow and multiply. Your home water tank is usually not a danger because you should keep the water heater thermostat at 60ºC (140ºF) to eliminate the risk of Legionella contamination.
The danger comes from inhaling microscopic water droplets containing the bacteria, and it can be all around you. The closer you are to the contaminated plume, the more likely you are to be hit.