How does a person get legionnaires' disease?

Water supplies. This is very rare, but typically from stagnant water supplies like collecting cistern on rooftops.
See below. Legionaires' disease is acquired by inhalation or microaspiration of the organisms into the lungs. The most common mode of spread is when contaminated water is aerosolized by water-cooled heat rejection devices such as air conditioning cooling towers, whirlpool spas, shower heads, water misters and some respiratory therapy equipment. It is not spread from person to person.