In the first half of the twentieth century, a strange and always-fatal illness spread among the Fore people of the Papua New Guinea highlands, and it spread almost entirely among women and young children.
Victims first grew unsteady on their feet, then developed tremors and lost coordination. As the disease advanced they could no longer stand, feed themselves, or control their movements. Some sufferers broke into bursts of uncontrollable laughter. The Fore called it kuru - “to shake.” Once symptoms appeared, death usually followed within a year.
For a long time the cause was a total enigma. Kuru didn’t behave like any familiar infection. It produced no fever, no inflammation, no sign of an immune response. Yet it clearly occurred in clusters, running through families and communities in a pattern that looked almost hereditary but didn’t obey the rules of inheritance. Researchers who travelled to the region in the 1950s found a society being damaged, with some villages losing so many women that the balance of the population was significantly distorted.
The breakthrough came from an unlikely direction: local funerary custom. As an act of respect and mourning, the Fore consumed the bodies of their dead. The pattern of who ate what proved informative. Women and children typically consumed the brain and internal organs - the tissues where the infectious agent was most concentrated - while adult men ate mostly muscle and were largely spared.
But what was the agent? Whatever it was contained no DNA or RNA, withstood boiling, resisted formaldehyde, and defied classification within any existing category of pathogen. It could incubate silently for years, sometimes for decades.
The answer, pieced together over a generation of research, upended biology: the culprit was a prion, a normal cellular protein folded into an abnormal shape, which then coaxes neighbouring proteins to misfold in turn - a self-propagating chain reaction that gradually turns the brain spongy and riddled with holes. The idea that a protein alone, with no genetic material, could act as an infectious agent was so radical that it met fierce resistance but eventually earned Nobel Prizes, first for demonstrating kuru’s transmissibility and later for the prion concept itself.
Once the Australian administration and changing custom brought the funerary practice to an end around the late 1950s, new infections began to dry up. But because prions can lie dormant for so long, occasional cases continued to surface decades later, the last victims carrying an exposure from childhood into old age. Scientists discovered that the epidemic had left a genetic fingerprint: a protective variant in the prion gene had become more common in the population, evidence of natural selection acting on humans within living memory.