Taenia solium, a parasitic tapeworm, infects the brain tissue with cysticercus, the larval form of its life cycle.
Encysted larvae that are burrowed within undercooked swine tissue are ingested. As the larvae hatch within the small intestines, they mature into adult tapeworms that continue to shed eggs. These eggs, if ingested, hatch to form larvae that migrate through the blood stream into the rest of the body, penetrating the body and encysting elsewhere. These can grow within the skeletal muscles (much like their encystment in swine tissue) or into the brain.
This is the most common parasitic disease of the nervous system and the main cause of acquired epilepsy for people in the developing countries.
I think having parasites growing in my brain would be one of the worst ways to die.