Final answer:
Legionella pneumophila is the bacterial pathogen found in water distribution systems, causing Legionnaires disease.
Step-by-step explanation:
The bacterial pathogen found in water distribution systems is Legionella pneumophila. This bacterium is responsible for causing Legionnaires disease and is commonly found in warm water pools, such as those in air conditioning units in large buildings. It can spread in aerosols, contaminating the water supply and causing outbreaks of the disease. The first known outbreak of Legionnaires disease occurred in a hotel in Philadelphia in 1976.