Hole in the Head or also commonly known as Hexamitiasis, is a parasitic in which lesions will appear over the head of the fish and can cause holes through the fish's head. In most cases, there will be erosion not only around the fish's head but also in various spots around its body as well.
Any fish that is infected with this disease will produce white, stringy poo, and their body coloration will go from being vibrant to a paler in appearance. They also will appear to have holes forming at the top of their head (can also be on the sides, underneath, or around their whole body) which will grow bigger if this is not treated promptly. As this parasite affects the fish even more, they will gradually eat less and less until they become emaciated. The most common fish to be infected by this disease are cichlids, however various carp related fish such as goldfish and koi can be infected just as easily.
Hexamitiasis is caused by the protozoan parasite genus Hexamita, which is present in the lower levels of the intestines of most fish (as they do not harm that location). Only when the fish's immune system becomes extremely weak is when the parasite can multiply and spread to the rest of its body. Sadly, as the parasite infects other key organs the fish will have no other fate than death (even with treatment). Since a large majority of fish already carry this parasite inside of their intestines, it is not technically considered to be contagious to others who are not infected by it.
Be aware though that just the fact of a fish having lesions or erosion on its head or body is a case of Hexamita, in some cases, malnutrition can cause the same symptoms which lead many to believe that their whole tank (or a group of fish) are infected.
Treatment and Medication
The primary medication that is known to treat this disease is metronidazole (often commonly called Flagyl). By using this medication in the infected fish's food will kill off the parasite from internal to external (thereby forcing the parasite out of the fish's body where it will die in just the water column). Another least effective way when the fish is refusing to eat would be to dose the medication inside of the water column (following the bottle's directions for exact dose requirements is necessary).
Due to the way that this disease spreads and affects the inhabitants, the best prevention methods are to always make sure that you quarantine anything that you place inside of your water (this includes any plants, inhabitants, decorations, and more). Thus, when you do let them adjust to your water and you truly see how they are acting, eating and if they have any symptoms or signs, we will either start to get a better understanding if they are infected or learn about what possible other diseases they might have.