i recommend you get the basics from wiki.
http://en.wikipedia.org/wiki/Hole_in_the_head_disease
based on what i read, hith is some kind of immuno malfunction caused
poor nutrition
inadequate nutrition(i.e not enough for its stress ful life)
poor digestion(hexamita parasite attach intestines of animal reducing nutrient absorption)
genetics.
edit: i really have no clue why i researched it, ive only had 2 cichlids in my life, maybe its a good mystery.