Also fascinating, but tell me, wouldn't subsequent tank bred fish, after numerous generations become immune? I understand that the vast majority of all Oscars sold are not wild caught, correct? Or does immunity not work this way?
Not necessarily. As an example, fish do not become immune to S. vortens. They can live in harmony when the numbers of pathogens are kept in check by a healthy fish, but if/when that host (your fish) comes under stress, those pathogens can multiply in numbers causing the fish to become very ill, very quickly.
The following article is fairly accurate, other than the use of the catch all term hexamita, which in freshwater fish has been identified as Spironucleus vortens.
http://www.fishchannel.com/fish-health/freshwater-conditions/hexamita.aspx
From a previous thread of mine on bloat ......
In a healthy fish S. vortens is commonly found in the flagellated stage in the lumen of the upper intestine where it is controlled by the immune system of the fish. In a stressed fish, the immune system becomes compromised, and these flagellates multiply unchecked causing serious localized damage. Once the damage is severe enough the intestinal lining is penetrated and the flagellates enter the blood stream causing systemic and organ infections, failure, and ultimately death of the fish.
There are many other stress factors that can take place in a glass tank, and it is typically those stress factors that trigger an outbreak of internal pathogens, usually Spironucleus vortens, that ultimately result in bloat conditions.
Hexamita Intestinalis is another catch all term to describe the various protozoa that trigger bloat conditions in tropical fish. In the vast majority of cases, those where clinical studies have identified the actual flagellates involved (specifically in cichlids), it has been Spironucleus vortens, not Hexamita or Octomitus species as previously believed. While it appears that much of the earlier identifications in ornamental species of fish may be erroneous, the overall treatment is pretty much identical.
So what may start out as an internal outbreak of S. vortens, could end as a secondary bacterial infection, resulting in large infected sores in the fishes head/body.
BTW - one of the most experienced discus breeders that I know raised his discus on pellets, some BW's on occasion, and only changed the water once a week. The juvies were all raised in hard water, with a pH of 8.0 and temps of 82F. I never saw a fish with HITH in any of his tanks.