While the jury is still out on much of the information surrounding the causes of HITH (freshwater) & HLLE (marine) it is clear that there can be several triggers, and what one hobbyist may experience with their fish, may have nothing to do with what the next person experiences with their fish.
There is no single cause of HITH, the cause is stress, and in many cases that stress does appear to come from the water chemistry such as what viejafish mentioned. The parasite that has been directly associated with HITH and freshwater fish species, is Spironucleus vortens. Myself, Matt, and Tom explain our personal opinions/theories as to why certain SA species are more prone to this ailment in the following past discussion. With the search feature broken this wasn't easy to find.
http://www.monsterfishkeepers.com/forums/showthread.php?548799-HITH-lATERAL-LINE-DISEASE-IN-CICHLA
In the case of blackwater species I always believed that those species found in acidic blackwater environments do not always cope well with various parasites/pathogens found in non blackwater environments. (such as found in many hobbyists tap water)
There is much more than just pH involved, and even tannins, various other natural turpines and chemicals found in indigenous plant matter may not only keep the pH buffered quite low in these natural blackwater environments, but in some instances (such as Terminalia catappa aka Almond leaves) some of these various indigenous plant matter are also known to produce antimicrobial activity that suppresses both gram positive & gram negative bacteria. Take those species out of that "protective" environment, and introduce them to paramaters outside their ideal range, and a fish that comes under stress (of any form) is going to be open for invasion from any number of pathogens that it would not normally encounter in the wild.
Enter the world of HITH, and S. vortens .....
