Kuru is a prion disease of human brain tissue. Prions are proteins that are incorrectly folded. They cause adjacent proteins to fold incorrectly, turning them in to new prions. The disease progresses slowly but eventually causes severe cognitive problems and death.

Kuru is endemic to the Fore people of Papua New Guinea. They have a funerary practice of eating the bodies of deceased community members. The practice has important religious and spiritual significance to them. Unfortunately, at some point in the past a member of the Fore spontaneously developed this prion disorder and their practice of funerary cannibalism provided a vector for it to spread in the community and become endemic.

https://en.wikipedia.org/wiki/Fore_people#Kuru_among_the_South_Fore

It's really very tragic. The very practice that people used to honor their deceased loved ones and protect the community from spiritual dangers turned out to be the vector for a deadly disease that almost wiped them out.

Edit: Apparently there are scholars that argue that dismemberment of the body was practiced during funerary rights, but cannibalism was not, and that the spread of Kuru might have occurred during a specific period of famine when cannibalism was practiced out of desperation.