Not usually, but in this magical setting I can believe cannibalism is slightly more common and you don't know how many humans the meat ate before you.
Well, no, that's not how that works. You catch a prion disease from eating someone that has a prion disease. The disease is extremely rare. And fatal. If you have a prion disease, you have, at best, a few years to live. Even the mad cow disease epidemic in Europe a few decades ago only reported a handful of cases of humans contracting the disease.
The likelihood of actually eating someone that has it are astronomically low. Hell, the gang is probably more likely to contract something from eating monster meat.
Pretty sure you get it specifically from eating the human brain. I might be wrong.
No, you get it from eating meat contaminated with it. The human brain does not naturally contain prions. Prions are fatal. If you have them, you will experience rapid mental deterioration and eventual death.
The reason everyone thinks eating human brains will infect you with prions is because of the viral (no pun intended) news story of the Kuru disease discovered among the Fore people of Papua New Guinea. They practiced funerary cannibalism, spreading the disease among their people for decades, but they stopped practicing cannibalism in the 60s and stopped seeing any cases of the disease after 2010.
Under normal circumstances you would not contract a prion disease unless consuming the meat of someone who already had one. Very very rarely you can spontaneously develop it (which is believed to be how the Fore people got it), but this is even rarer than getting it through consumption and typically only happens to older people. If someone is infected with it, then the brain contains the highest concentration of prions, providing the highest risk of contracting it if consumed, but again, that specifically relies on the person being consumed already having it. You wouldn't get it just from eating a perfectly healthy brain.
I think the women are standing farther from the viewer. According to their official stats, the Idolmaster Anime producer is only 7 inches taller than Ritsuko.
