I don't think that there is much doubt at this point, based on my understanding of the literature, that hexamita parasites cause the head/body lesions that we call HITH. The real question at this point is what the aetiology of the disease is. In other words, what makes a fish susceptible to HITH and how does the disease progress? Given that so many factors have been positively associated with HITH, I suspect that it's a safe bet that any factor that immuno-compromises a fish might be positively associated with HITH. This includes amorphous "stress" in addition to all the other factors that have been mentioned in this thread and the article I've linked above.
This means that, if you have poor water quality, your tank is a ticking time-bomb for HITH.
To avoid: do your best to keep your fish from becoming immuno-compromised by your fish-keeping methods.
To cure an infection: Improve water quality and treat for parasites. Improving water quality does not necessarily mean frequent large water changes.