I hadn't even thought of Legionella, but that's true, keeping moderately warm water around can create the perfect environment for bacteria to thrive.

From the first link Doug references above:

Quote:
...recommends for hot water storage cylinders that the whole contents should be heated to 60oC for one hour each day to prevent growth of Legionella bacteria in the cooler water at the bottom of the cylinder. Briefly raising the water temperature to 60oC is not effective.

If pre-heated water between 20oC and 45oC containing Legionella is subsequently passed through a combination boiler, its temperature does not usually exceed 60oC and it is held at that temperature only for seconds before being distributed and used.

So, even if your system has a boiler, the water may not be safe because the water is not heated high enough or long enough by the boiler to kill off Legionella before it is sent to your taps and shower.