It was a prion disease that was only contracted in the Fore people. The only way to get it was to be born with it or eat someone with the disease. So once people there stopped cannibalism, the disease began to disappear.

I think it is something that happens naturally every so often, and eating brains is a trigger for it if not the only one. Like cows will naturally get mad cow disease every so often naturally or something like that it might be different than the spread stuff.