Scientists have cracked the secret of how the rogue proteins thought to cause vCJD and other brain diseases are able to form different strains.
Prions are responsible for brain disease
Researchers had assumed this must only be possible if these proteins - prions - had some form of genetic content.
But US researchers have shown that they are made up solely of protein, and that new types result simply from prions twisting into new shapes.
The research is published in the journal Nature.
It suggests that the ability of prions to misfold into new formations - or strains - accounts for their ability to trigger different diseases.
As well as CJD in humans, they are thought to be responsible for BSE in cows, and scrapie in sheep.
The researchers, from the University of California at San Francisco and Florida State University, believe that once a prion has folded into a new shape, it acts as template for others.
This produces a chain reaction that allows infection to spread.
They hope their work will shed light on what remain relatively poorly understood neurological diseases.
Researcher Dr Jonathan Weissman said: "I would say this puts to rest any question about whether the protein-only prion hypothesis as a general principle is true.
"And it also establishes that prion strains can be accounted for solely by the ability of the protein to misfold into more than one conformation."
The latest work was based on experiments on prions found in yeast, which are similar to those that damage human and other mammalian brains.
The Californian scientists developed a technique to split a yeast prion into two different strains by exposing it to different temperatures.
Meanwhile, the Florida team isolated three different strains of yeast prion - each of which originated from the same protein molecule.
The Florida scientists showed that prions acted in much the same way in yeast as they did in mammals.
Introduced into healthy cells, the misfolded proteins made contact with molecules identical to the proteins from which they were originally made, and caused them to warp into exactly the same shape.
This led to the disruption or alteration of normal cell function.
In mammalian infections, prions trigger protein clumping that can kill brain cells, whereas yeast prions merely alter a cell's metabolism and are not deadly.
A UK Department of Health spokesman said: "This research will be given to the government's expert committee on these issues, the Spongiform Encephalopathy Advisory Committee (Seac), at its next meeting."