I had already mentionned how her disease worked in author's notes, but I wanted to at least mention it in the comic so it would be official.
Katie is now the second character to officially have a car! (After John)
Typically evolution installs fail safe devices to keep one anatomical feature from breaking the others, but they don't always work.
Fairies are immune to the dust, but she's only 1/4 fairy, she has two copies of the human gene for the relevant receptor. Brains are pretty flexible so she's adapted well enough not to be in a constant state of delirium, but it has side effects.
A bad education probably didn't help. It's canon that Jean-Marie is representative of the family, it was a pretty hostile environment.