Britain could be on the brink of another CJD crisis as scientists fear that many more people are carrying the disease than previously thought.
A study found a patient who contracted the human form of BSE through a blood transfusion had a crucially different genetic make-up to any known previous victim.
The patient's gene pairing is found in about half the general population.
In these individuals, the incubation period of the disease may be much longer - raising the possibility of a major epidemic of cases occurring at some unknown time in the future.
It had been assumed that just more than a third of the population could develop the disease.
There could be a large pool of symptomless people who do not realise they are carrying the infectious prion protein responsible for both the cattle brain disease BSE and vCJD.
There is a chance they could transmit the agent to others via blood transfusions or contaminated surgical instruments.
Professor James Ironside, from the National CJD Surveillance Unit in Edinburgh, who took part in the study, said: "This finding has major implications for future estimations of numbers of vCJD cases in the UK, since individuals with this genotype constitute the largest genetic subgroup in the population.
"It's absolutely possible that there may be a new epidemic, because the cases we've seen so far may only be those who are unusually susceptible or have the shortest incubation periods.
"I'm not in the business of scaremongering, but quite clearly the idea that this problem is on the way out is unfortunately not the case at all."
Cases of vCJD peaked in 2000, when 28 deaths were reported. Since then the trend has been generally downward, with 17 cases in 2002 and 18 last year.
Scientists have estimated that almost 4,000 people could be harbouring vCJD, based on studies of appendix samples.