There is a great post about legionaires disease over on another forum, it is actually a risk with WFP, not a snowballs chance in hell of it happening in the winter though (allegorical, I like that
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It isn't the storing of the water itself, it is also linked with the temp of the water being stored, has to be a certain temperture for the bacteria to develop.
If proper procedures are followed it won't happen, ie, changing filters regularly, keeping your water treatment out of direct sunlight, and so on.
It's very unlikey to actually happen though, WFP has been around for years, if it had caused a case of Legioaires disease somewhere in the world, I'm sure we would have heard of it
Ian