A real human brain is displayed in the 2001 interactive exhibit “Brain: The World Inside Your Head” at the Smithsonian’s Arts and Industries Building in Washington. (Andrea Bruce Woodall/The Washington Post)

The sickness spread at funerals.

The Fore people, a once-isolated tribe in eastern Papua New Guinea, had a long-standing tradition of mortuary feasts — eating the dead from their own community at funerals. Men consumed the flesh of their deceased relatives, while women and children ate the brain. It was an expression of respect for the lost loved ones, but the practice wreaked havoc on the communities they left behind. That’s because a deadly molecule that lives in brains was spreading to the women who ate them, causing a horrible degenerative illness called “kuru” that at one point killed 2 percent of the population each year.

The practice was outlawed in the 1950s, and the kuru epidemic began to recede. But in its wake it left a curious and irreversible mark on the Fore, one that has implications far beyond Papua New Guinea: After years of eating brains, some Fore have developed a genetic resistance to the molecule that causes several fatal brain diseases, including kuru, mad cow disease and some cases of dementia.

The single, protective gene is identified in a study published Wednesday in the journal Nature. Researchers say the finding is a huge step toward understanding these diseases and other degenerative brain problems, including Alzheimer’s and Parkinson’s.

The gene works by protecting people against prions, a strange and sometimes deadly kind of protein. Though prions are naturally manufactured in all mammals, they can be deformed in a way that makes them turn on the body that made them, acting like a virus and attacking tissue. The deformed prion is even capable of infecting the prions that surround it, reshaping them to mimic its structure and its malicious ways.

The prions’ impact on their hosts is devastating and invariably fatal. Among the Fore, the prions riddled their victims’ brains with microscopic holes, giving the organ an odd, spongy texture. In cattle, prions cause mad cow disease — they are responsible for the epidemic in Britain of the late ’80s and ’90s that required hundreds of thousands of cattle to be destroyed. They have been linked to a bizarre form of fatal insomnia that kills people by depriving them of sleep. And they’re the source of the degenerative neurological disorder Creutzfeldt-Jakob disease (CJD), characterized by rapid dementia, personality changes, muscle problems, memory loss and eventually an inability to move or speak.

The vast majority of prion-diseases are “sporadic,” seemingly appearing without cause. But a lead author of the Nature study, John Collinge, said in an interview with Nature that a portion of cases are inherited from one’s parents, and an even smaller percentage are acquired from consuming infected tissue. Variant CJD, often called the “human mad cow disease,” is caused by eating beef from infected cows.

Prions are especially insidious because there’s no way of stopping them, science writer D.T. Max, author of a book on prions and fatal familial insomnia, told NPR in 2006. In the hierarchy of pathogens, they’re even more elusive and difficult to quash than a virus. They can’t be treated with antibiotics or radiation. Formalin, usually a powerful disinfectant, only makes them more virulent. The only way to clean a prion-contaminated object is with massive amounts of extremely harsh bleach, he said. But that technique isn’t helpful in treating a person who has already been infected.

The study by Collinge and his colleagues offers a critical insight into ways that humans might be protected from the still-little-understood prions. They found it by examining the genetic code of those families at the center of the Fore’s kuru epidemic, people who they knew had been exposed to the disease at multiple feasts, who seemed to have escaped unscathed.

When the researchers looked at the part of the genome that encodes prion-manufacturing proteins, they found something completely unprecedented. Where humans and every other vertebrate animal in the world have an amino acid called glycine, the resistant Fore had a different amino acid, valine.

“Several individuals right at the epicenter of the epidemic, they have this difference that we have not seen anywhere else in the world,” Collinge told Nature.

That minute alteration in their genome prevented the prion-producing proteins from manufacturing the disease-causing form of the molecule, protecting those individuals from kuru. To test whether it might protect them from other kinds of prion disease, Collinge — the director of a prion research unit at University College London — and his team engineered the genes of several mice to mimic that variation.

When the scientists re-created the genetic types observed in humans — giving the mice both the normal protein and the variant in roughly equal amounts — the mice were completely resistant to kuru and to CJD. But when they looked at a second group of mice that had been genetically modified to produce only the variant protein, giving them even stronger protection, the mice were resistant to every prion strain they tested — 18 in all.

“This is a striking example of Darwinian evolution in humans, the epidemic of prion disease selecting a single genetic change that provided complete protection against an invariably fatal dementia,” Collinge told Reuters.

The Fore aren’t the only people to demonstrate prion resistance. More than a decade ago, Michael Alpers — a specialist on kuru who has studied the Fore since the 1960s and was a co-author of the Nature study — conducted similar research on prion protein genes in humans worldwide. In a study published in Science, he found that people as far-flung as Europe and Japan exhibited the genetic protection, indicating that cannibalism was once widespread and that prehistoric humans probably dealt with waves of kuru-like epidemics during our evolution.

But the gene found in the Fore is special because it seems to render mutant prion-producing proteins (the kind that would be passed down from one’s parents, causing inherited prion diseases) incapable of producing any kind of prion whatsoever. It also stops the wild-type protein — the phenotype that most people have — from making malformed prions.

Scientists say that the benefits of this discovery don’t stop at prion diseases, which are relatively rare — only about 300 cases are reported each year in the United States. According to Collinge, the process involved in prion diseases — prions changing the shape of the molecules around them and linking together to form long chains called “polymers” that damage the brain — is probably responsible for the deadly effects of all kinds of degenerative brain illnesses: Alzheimer’s, Parkinson’s and dementia chief among them.

According to the World Health Organization, there are 47.5 million people worldwide living with dementia. An additional 7.7 million are diagnosed each year.

If Collinge and his colleagues can understand the molecular mechanisms by which prions do their work — and how the prion-resistant gene stops them — they might better understand the misshapen proteins that are afflicting millions with those other degenerative brain illnesses.

Eric Minikel, a prion researcher at the Broad Institute in Cambridge, Mass., who was not involved in the study, was impressed by the finding.

“It is a surprise,” he told Nature. “This was a story I didn’t expect to have another chapter.”