There are ‘good’ and ‘bad’ bacteria all around you every day in the atmosphere, on your food and in water. Legionnella is one of the bad guys found in water (rivers and lakes) and water droplets. We normally here about it from outbreaks in poorly maintained water systems where it grows when conditions are warm (20-30C) and stagnant. Below 20C is grows very slowly and above 40C, it is either ‘out competed’ by thermophilic bacteria (40-60C) or killed off by heat (eg hot water above >45C).
Life is never risk free – in our opinion this is a very rare risk present in any forms of water, is rare in composting and should be even rarer in a well maintained hot composting system.
You can find more background on legionnaires at NHS here: http://www.nhs.uk/conditions/Legionnaires-disease/Pages/Introduction.aspx
P.S. We think the article title ‘wash off compost’ is hopelessly misleading – you can’t wash compost and as legionnella is water borne; adding water is likely to make the situation worse!