They have resorted to cannibalism

At many times in history, when people are trapped and hungry, they have resorted to cannibalism: a town under siege has no option but to eat its dead in order to survive. Yet there are a few groups for whom cannibalism is the norm. However, even when not taboo, cannibalism is not always safe. Prion diseases are easily transmissible between individuals via cannibalism. These are similar to mad cow disease, and cannot be prevented by thorough cooking. Prion diseases cause the brain to deteriorate by an increasing number of tiny holes, making it appear like a sponge. Mental and physical abilities deteriorate until the victim experiences a painful and confused death. The Fore people of Papua New Guinea suffered in this way until the 1950s. They traditionally dismembered and ate relatives as part of their ritual funerals, viewing a dead person as a food source. A person infected with a prion disease would become increasingly disabled until the disease killed them, and when eaten during their funeral, would pass the disease on to others. It was seen as a curse rather than a disease, and so the Fore people took no steps to prevent infection. In this way the disease spread quickly to epidemic proportions until the local government banned cannibalism.