Discovery of the pathogenic relationship between amoebas and Legionella bacteria--a key step in understanding how this bacteria infects mankind. Legionella bacteria, along with amoeba, live in the organic contamination on the surfaces of water systems. Rowbotham demonstrated that Legionella grow inside another cell, specifically amoebas. Ordinarily amoeba eat bacteria and organic contamination, and in the process kill those bacteria.

It was later understood by others that when amoeba eat Legionella the bacteria become encapsulated, and continue to grow inside the amoeba until they are either released from the diseased amoeba into the water or a protective biofilm composed of mostly complex carbohydrate matrices containing the Legionella is released. In either form Legionella can be aerosolized and inhaled by a human host.

Once in the respiratory tract of humans, Legionella are ingested by macrophages and possess the unique ability to replicate inside the phagosome within the alveolar macrophages, which act in a protective manner for this specific bacteria. The key to the survival of Legionella within the macrophage, and thus their virulence, is the ability of Legionella  to prevent phagosome lysosome fusion and subsequent destruction.

(Thanks to Juan Weiss for this reference and its interpretation.)