The prions originate in the brains. Everyone has prions. Everyone's own personal prions are benign to themselves but act like a virus in others. The cannibals of Paupau New Guinea, in specific the males (because women are not allowed to eat the brains) eat the brains of their enemy and doom themselves to Jacob-Croitzfield disease (human scrapie) by the time they are 40. I don't know if they have managed to convince the people not to eat the brains of their enemies in these last 20 years but the information I had on them is about 20 years old and none of the men were living past 40.
Do not eat brains of any species. The Mad Cow Disease was caused because some idiot wanted cheap food to feed cattle and decided to use ground up sheep parts mixed with enough grain and molasses to fool the cows into eating like an omnivore. The scrapie was spread to the cattle in England because of this.
Actually, you have a bit of the information about kuru (I assume you mean kuru, since you are speaking of a Creutzfeldt-Jakob type disease (CJD) in Papua New Guinea) wrong. Women and children were actually more prone to acquiring the disease. The Fore people of Papua New Guinea consumed deceased relatives, in order to return the deceased person's 'life force' to the living. Men were less likely to get the disease because they had seniority, and thus ate the choicer pieces, mostly muscle with little nerve tissue. The women (and children) got the less muscled parts, which often led to them consuming spinal or brain tissue, where the misfolded proteins were more concentrated. It acted like some horrible, endless loop of disease, because by consuming those who died of kuru, it spread kuru to otherwise healthy individuals, who then died themselves. Plus, it had a very, very long incubation period.
Microbiology, disease, and whatnot are rather faciniating to me. So hopefully that clears up a few misconceptions.
You are right that all animals have prions. But most are correctly folded into a PrPC structure. When the protein is damaged in some way, it can change its form, which is called PrPSc These proteins act as templates, causing normal ones to fold into the infectious/disease causing form.