Rats can get in anywhere.
I like you Graham Gould. You make me laugh.
Unless there's a rat called Harry Houdini - with Weils disease - who can unscrew the tops of water bowsers, then cleverly perch themselves in a safe position and wee, without falling in and drowning, then I reckon the chances are NIL. However, I'd pay good money to watch it, if anyone knows of such a rat!
And, if rats get everywhere, then they can wee in your house's water tank. The water might be chlorinated, but I haven't studied what germs that will kill. I mean it's not ruddy bleach is it. Tap water doesn't 'Kill all germs dead', like the 'blue stuff' claims.
As for Legionellosis - I hear WFP window cleaners are dropping left right and centre from it!
If you read in another forum that Weils and Legionnaires disease was a realistic danger to WFP users, AND you actually believed it, then send me £100.00 and I promise I will send you a vaccine that you can drink (it tastes just like orange juice), and you will never ever, contract Weils or Legionaires disease through using pure water.
As an afterthought. It must be a far greater danger from disease from standing in dog and cat poo, then walking it up and down your ladder!