Skullfire

The Skull Fire was an incredibly rare transmissible degenerative brain disease, almost unheard of in modern times. 
It was known to the Eja as Capúl hęz (skull fire), where it gained its name after being attested in other tribal paintings and records.
One period in which it was notably prevalent was during the later Predecessor age. Cannibalistic tribes such as the Eja would gain it from eating the brains of their victims, and pass it on to their children. 
It caused madness and unpredictability as well as shaking and slurred perception.