A disease linked to cannibalism has given clues about how long mad cow disease (BSE) can lurk in the human body before it develops into vCJD.
vCJD, like kuru, is a prion disease
A University College London team said it could take 50 years for vCJD, the human form of the disease, to develop.
They studied Papua New Guineans with a related condition - kuru disease, which is contracted through cannibalism.
In The Lancet, the team said people with a certain genetic make-up risked long-term vCJD incubation.
Exposure to BSE (bovine spongiform encephalopathy) in the UK has been widespread, although just 160 vCJD (variant Creutzfeldt-Jakob) patients have been identified, leading scientists to investigate why more people have not been affected so far.
Kuru disease, like vCJD, is a prion disease. Prions are mutated proteins.
It reached epidemic proportions in some Papua New Guinea communities early in the 20th Century.
Eating dead relatives as a mark of respect and mourning was ritual practice until it was banned in the 1950s.
In the study, 11 patients with kuru were identified between July 1996 to June 2004, with the last one born in 1959.
Although it was not possible to know the exact date the patients contracted kuru, the possible incubation periods ranged from 34 to 56 years.
The researchers believe the incubation period for BSE prions in humans could be even longer than that seen in kuru because infection between different species typically takes longer to develop than one passed within the same species.
Professor John Collinge, who led the study, said vCJD patients identified so far "could represent a distinct genetic subpopulation with unusually short incubation periods for BSE".
He said a human BSE epidemic might have a number of phases, and added: "Recent estimates of the size of the vCJD epidemic based on uniform genetic susceptibility could be substantial underestimations."
An editorial in the Lancet stated: "The eventual size of the vCJD epidemic remains uncertain.
"The number of infected individuals is still unknown.
"By investigating kuru, the only known example of a major epidemic of a prion disease, we will begin to narrow our present uncertainties about vCJD.
"Any belief that vCJD incidence has peaked and that we are now through the worst of this sinister disease must now be treated with extreme scepticism."