Kuru is a fatal brain disease caused by an infectious misfolded protein called a prion. It was once epidemic among the Fore people of Papua New Guinea, where it spread through the ritual consumption of deceased relatives’ tissue, particularly brain. Sometimes called the “laughing disease” because of the uncontrolled laughter it could trigger, kuru killed roughly 25% of the female population in some Fore communities during the 1950s and 1960s.
How Kuru Spreads
Unlike bacteria or viruses, prions are not living organisms. They are normal brain proteins that have folded into the wrong shape. Once a misfolded prion enters the body, it acts like a template, converting healthy proteins into copies of itself. These abnormal proteins clump together and gradually destroy brain tissue, leaving it riddled with tiny holes that give it a sponge-like appearance under a microscope. There is no immune response that can stop the process, and no treatment that can reverse it.
Kuru spread among the Fore through funerary cannibalism. When a community member died, relatives would consume parts of the body as an act of mourning and respect. Men rarely participated in these feasts, which is why the disease overwhelmingly struck women and young children. Children were exposed because they accompanied their mothers and ate contaminated tissue, including brain. This pattern initially puzzled researchers, who considered whether the disease might be genetic because it seemed to run in families and along gender lines. The cannibalistic transmission route explained both the family clustering and the lopsided gender ratio.
Symptoms and Stages
Kuru progresses through three distinct stages over the course of roughly 12 months, though some patients decline faster. The disease primarily attacks the cerebellum, the part of the brain responsible for coordination and balance, which is why movement problems dominate the clinical picture rather than memory loss.
Ambulatory Stage
The earliest sign is a subtle unsteadiness when walking. This progresses to visible difficulty coordinating the legs and trunk. A characteristic tremor, described as a fine shivering through the whole body, gives the disease its name: “kuru” means “to shake” in the Fore language. Cold temperatures make the shivering worse. People in this stage may also curl their feet and claw their toes in an effort to stay balanced. Speech begins to slur, and hand movements become imprecise. Emotional instability is common, including episodes of spontaneous, uncontrollable laughter that led outsiders to call it the “laughing disease.”
Sedentary Stage
This phase begins when a person can no longer stand without support and ends when they can no longer sit upright without help. The tremor, slurred speech, and coordination problems all worsen. Jerky, involuntary eye movements appear. Muscle stiffness and abnormal postures become more noticeable, and reflexes grow exaggerated.
Terminal Stage
In the final stage, the person is bedridden. Swallowing becomes difficult, and incontinence develops. Patients may appear unresponsive to their surroundings yet remain conscious. Involuntary writhing and jerking movements continue. Unlike other prion diseases such as Creutzfeldt-Jakob disease (CJD), severe dementia is not a prominent feature of kuru, though some cognitive decline can occur. Death typically results from pneumonia, infection, or the inability to eat and drink.
An Extraordinarily Long Incubation Period
One of the most remarkable features of kuru is how long it can hide in the body before symptoms appear. The typical incubation period ranged from about 10 to 13 years, but some cases surfaced more than 50 years after the last possible exposure. The Fore ceased cannibalistic practices in the late 1950s after Australian authorities and missionaries discouraged them, yet new cases continued to appear for decades afterward. These late-onset cases proved that prions can silently replicate in the body for an entire human lifetime before causing detectable damage.
What Happens Inside the Brain
When researchers examine the brains of kuru patients under a microscope, they find widespread clusters of misfolded prion protein that form distinctive round deposits known as “kuru plaques.” These plaques are unusually prominent and found throughout the brain, particularly in the cerebellum. The surrounding tissue shows the sponge-like holes characteristic of all prion diseases. This pattern of damage, concentrated in the brain’s coordination center rather than its memory and reasoning areas, explains why kuru patients lose control of their bodies before their minds.
The same type of plaque later turned up in cases of CJD that were accidentally transmitted through medical procedures, reinforcing the link between kuru and the broader family of prion diseases.
How Kuru Changed Science
Kuru was the first human prion disease ever identified, and its study reshaped how scientists think about infectious agents. In the 1950s, American researcher Daniel Carleton Gajdusek traveled to Papua New Guinea and began investigating the epidemic. He and his colleagues demonstrated that kuru could be transmitted to chimpanzees, proving it was infectious rather than purely genetic. This work earned Gajdusek the Nobel Prize in Physiology or Medicine in 1976. The discovery that a protein, rather than a virus or bacterium, could act as an infectious agent was initially met with deep skepticism but eventually became one of the most important breakthroughs in modern neuroscience. It connected kuru to CJD in humans, scrapie in sheep, and later to bovine spongiform encephalopathy (mad cow disease) in cattle.
A Genetic Legacy of Survival
The kuru epidemic was so devastating that it left a mark on the Fore people’s DNA. Researchers studying survivors and their descendants discovered a genetic variant, known as G127V, in the gene that codes for prion protein. This single amino acid change appears to completely prevent prion disease. In laboratory experiments using mice engineered to carry the human version of this variant, the animals were entirely resistant to infection with both kuru and CJD prions. The variant was found only in people from the kuru-exposed region of Papua New Guinea, and genetic analysis confirmed it was under strong positive selection during the epidemic, meaning individuals who carried it were far more likely to survive and have children.
Even in its heterozygous form, meaning a person carried just one copy of the protective variant alongside one normal copy, it provided strong protection against kuru. The misfolded protein encoded by this variant is not only resistant to conversion itself but can actually inhibit the spread of misfolded prions in a dose-dependent way, slowing or stopping the chain reaction that destroys brain tissue.
Current Status
Kuru is essentially extinct. After funerary cannibalism ended in the late 1950s, the number of new cases steadily declined over the following decades. The last known deaths occurred in the 2000s, representing individuals with exceptionally long incubation periods who had been exposed as children before the practice stopped. No new cases have been reported in recent years, and with the transmission route eliminated, the disease is not expected to return. Its legacy, however, endures in the scientific understanding of prion diseases and in the protective genetic adaptations still carried by the Fore people.