Scientists are a step closer to understanding how rogue proteins, which cause diseases such as vCJD, move between species.
Prions have barriers which should protect them
The study in Molecular Cell may help explain how humans develop vCJD after eating BSE infected meat.
Scientists had not understood how the proteins - prions - from one species could breach the defences of another.
But US researchers found a way for the prions to take on some characteristics of the "victim" species and sneak in.
Prion diseases, also known as transmissible spongiform encephalopathies, are a group of infectious neurodegenerative disorders.
They include Creuzfeldt-Jakob Disease (CJD) and vCJD in humans, bovine spongiform encephalopathy (BSE) in cows, and chronic wasting disease in deer and elk.
The abnormally shaped prions which cause the diseases are thought to act as a template which binds to normally shaped proteins forcing them to change shape into an abnormal, disease-causing, form.
This is a relatively straightforward process in animals from the same species.
But scientists did not understand how the rogue prions converted proteins in different species, as each species' prions have their own barriers, and should not be able to be altered by "invaders" prions.
In lab tests, researchers at Case Western Reserve University School of Medicine were able to overcome these natural barriers.
They were able to do this by introducing a tiny number of fibres from a hamster prion into a mouse prion, creating a new prion strain.
Unaltered mouse prions were unable to latch onto hamster prions.
But the new version of the mouse strain was able to overcome the species barrier and could convert the hamster proteins.
The researchers suggest this kind of mutation could be how the prions cross between species naturally.
Even though this was test-tube research and the new strain has not yet been shown to be infectious in animals, the scientists involved say it offers important clues as to how prion diseases cross between species.
Writing in Molecular Cell, the researchers led by Dr David Vanik, said: "The transmission properties of vCJD are very similar to those of BSE prions, indicating that vCJD represents a strain of human prion that emerged by infection with BSE prions."
They added that the apparent ability of prions to cross the species barrier "should be taken into account in any assessments of risk factors for the transmission of animal prion disease into humans."
Dr Jean Manson, head of the TSE division at the Institute for Animal Health in Edinburgh, told BBC News Online: "This study does put forward a possible mechanism for infection.
"But the researchers have not yet shown that the product produced in their system is infectious to animals. Until this can be shown the mechanism cannot be verified."
There have been 139 deaths since vCJD emerged in the UK nine years ago. Scientists have estimated that the worst of the infections could be over after the disease appeared to hit a high point in 2000 with 28 deaths, before falling.
Seven people thought to have the incurable disease are still alive.