There were a lot of great ideas here, and I'm super excited for these plots to unfurl.
Just some thoughts/ideas to add to the mix:
maybe I've just read and watched too many zombie/fantasy junk, but perhaps the disease could have been a replication of a really old curse made not only with just a crazy potion, but maybe involved runic spells or some serious blood magic. So maybe the cure could involve needing the blood of pureblood wizards as part of its ingredients. As for Healers, they'd probably have a record of the antidote, but because of blood magic involved, they'd have to be careful developing it since it's rather shady territory.
Since all purebloods are somehow related (at least heavily implied in the canon), they'd all have the cursemaker's blood in them, so maybe they quickly develop an immunity and become well. Meanwhile, half-bloods and muggle-borns are stricken with the disease much longer. In a way, this would seem like the Craze DE is finishing the task his master would have done if hadn't been killed. And this would fuel more of the pureblood resentment since they'd get well sooner while others didn't. As for Elementals, since they're more knowledgeable about runic and blood magic, they can probably counter it easily, or simply morph to purge the curse out of their system.
...anyway, I'm done being crazy. >>
And as for the anti-pureblood sentiment, I'm offering the Emersons as part of this political/class schism. It's such a prime moment for them take revenge on all the other pureblood families they've been in conflict with all these generations.