HITH (Hole In The Head) is best explained as a little mircoscopic creature that lives in your gravel or sand.

When the water gets dirty or your filters cant handle the bio-load of the fish this little parasites attach them selfs to a fish and eat into there head. You will first see signs of this if the holes are little pin sized ones, if you can see white in the pin holes then its the sign that HITH is about to begin on your fish.

If this is left alone the small holes will then fall in to create one large hole around the fish's head, this will happen all over there head as well.

How to fix it: There are plenty of chemicals out there that "claim" to work with HITH but they are dangerous to the fish and also to other fish in the tank and can be really confusing to use as well.
The best possible way to fix this is by water changes. My Oscar was battling HITH for about 3 months or so and I just couldnt get rid of it. I then started to read up on it and found out the best possible way to get rid of HITH was to just keep changeing the water.

I started to change the water every 2nd day, but only changeing 30% and adding 1 teaspoon to every bucket of water I added into the tank, so about 1 spoon to every 4-5g (9ltr's).
I did the water changes by vacumeing the gravel as I found out this parasite lives in the gravel and then comes out to attack a host (Fish) when the water gets dirty.

By about the the 7th day or so the holes were gone and my Oscar was fine. To prevent this from happening again I upgraded my filters as well as I found out it was because my filters were crap and they just couldnt handle the load of the fish in the tank.
I went out and got a canister filter and a Fluidized bed filter as well and my tank has never been more clean and my fish more healthy.
Also remember its one Oscar Per 50g of water if you have any more then there is no chance of getting rid of HITH.

Any questions then just ask away!

Ive just reading a bit about this disease as well and found
"Hole in the Head disease is caused by Hexamita, a flagellated protozoan that is found in the gastrointestinal tracts of many fish species. Even healthy fish can carry Hexamita. If the water quality drops or if your fish is weakened by some other problem, Hexamita can spread from the intestines via feces in the water and attack the outside of the fish. A small sore will form above the eyes of the fish and eventually grow in to a large hole. Large fish species are more prone to hole in the head disease; especially large cichlids such as Oscars and Jurupari Earth Eaters."

"QUOTE"
(I started to change the water every 2nd day, but only changeing 30% and adding 1 teaspoon to every bucket of water I added into the tank, so about 1 spoon to every 4-5g (9ltr's).
I did the water changes by vacumeing the gravel as I found out this parasite lives in the gravel and then comes out to attack a host (Fish) when the water gets dirty.)

Teaspoon of what? :?:

Ten to one they meant salt. Though hexamita as a cause for hith is still waiting to be proved, unless there has been some head way.