In a former life I was a biologist, but not in this area, so any advice from me is worth exactly what it cost.
My understanding of highly host-specific diseases was they are found in the wild in primarily a single species. In the confines of a tank, with GOK what general water conditions but we can assume some non-trivial degree of pollution, some stress and associated immune suppression, and given someone leaving the dead in the tank to be consumed at least in part by their tankmates, I would expect to see cross-species transmission of such parasites. Not large-scale, and perhaps not with same outcome as most visible NTD in Neons, but I do not believe the species barrier to be absolute - this is not a three-stage life cycle such as black spot, this is single-host direct transmission.
AIDS was after all in some other primate first. And prions/infectious proteins or whatever they are called these day have been know to jump species, and they are not even viruses or bacteria. Those I did have some working knowledge on.
In confined, more or less unheathy, environments, species specificity is luxury, not a safe limit.
The best ref I know on the web (human, not fish, but it does sho the life cyle well) is:
http://www.stanford.edu/class/humbio103/parasitepages/ParaSites_2002/microsporidiosis/index.HTM