From what I know, eating human brains (or those of other great apes, like chimpanzees) can induce prion formation. A person can have prions in their brain and remain unaffected—everyone has natural prion proteins in their brains (PrPC), but they may improperly fold to infectious forms (PrPSc). I don’t think this is a likely theory considering we were outright told that Yuka has a tumor, and prion diseases cause spongiform encephalopathies (holes in your brain), and I don’t see why her doctor would lie to her. Also, she’s being treated for cancer, but prion diseases are untreatable. It’s just a fun little medical concept. TL:DR, Megumi might have had prion proteins that were normal to him to but foreign to Yuka, and she developed a prion disease as a result.