legionnaires disease?

The only thing concerning legionnaires disese that i know of applies to stored hot water (hot water tanks), where the temperature should be above 50 deg C or below 20 deg C in order to prevent legionella from forming. this should ideally be controlled by a cylinder thermostat rather than the boiler thermostat, as that way you can have different temperatures for the radiators and hot water without being scalded by the water.
This would not apply to a combi boiler as it is fed from the mains water supply.
It may be that the boiler has a pre-heat function for the hot water, or the flow switch may be stuck on.

Ian
February 2010