Thousands of people could be incubating variant Creutzfeldt-Jakob Disease (v-CJD) as a result of eating infected beef in the 1980s, say scientists, but it may take decades before the effects are clear. We may not even know what has happened until any epidemic is almost upon us.

"I draw comparisons with kuru, which affected cannibals in the Pacific," said Professor John Collinge, an expert in v-CJD and BSE at the neurogenetics unit of St Mary's Med- ical School in London. "There we saw long incubation periods of decades. Put on to that a species barrier from cows to humans, which prolongs the incubation period, and you would probably expect an average incubation period of 20 to 30 years."

He thinks that it is easy to draw the wrong conclusion from the two years of tests on 3,000 appendixes and tonsils from hospitals in Lanarkshire in Scotland, and the south-west of England. Those found none with the "infective" prion protein (a misshapen form of the body's own PrP protein, found in cells). By contrast, prions have been found in all of the 53 Britons who have so far died of the incurable brain disorder.

The tonsils and appendix are part of the body's lymphatic tissue, which has been shown to accumulate prions before the brain is affected. This study should thus offer some insight into how many people may be harbouring the incurable illness. "We knew those studies, though necessary, could produce no news or bad news," said Professor Collinge. "This result is no news."

Dr Sheila Bird, a medical statistician who has followed the epidemic's progress, said just one positive sample in the 3,000 would imply there were thousands of people incubating the disease. But if the prevalence is one in 100,000, scientists would not expect to find any in the 3,000. But that doesn't mean it isn't out there.

Another 15,000 tests are planned for tonsils and appendixes removed between 1996 and 1998, which will take a further two years, while tests on 2,000 "fresh" samples are also being carried out. But even those are too small to give a definite picture of how many might be incubating the illness

With v-CJD, the unknowns multiply endlessly. How long is the incubation period? It may be five years, or 20 years. (It could be both.) It may be that only people with a particular genetic make-up (shared by 40 per cent of the population) are susceptible, as all the victims have that genetic make-up - or it might be that people with that genetic composition (a tiny variation of one amino acid in the huge PrP protein) develop the disease more quickly.

Ever since Stephen Dorrell, then the Secretary of State for Health, told Parliament in March 1996 that there appeared to be a link between BSE and v-CJD, people have wanted to know what the eventual figure would be. Professor Richard Lacey predicted in the 1980s that millions of people would die of CJD - a prediction that shows no signs of coming true. In 1997 two British scientists, Dr Peter Smith - now a member of the government's advisory body on BSE and v-CJD - and Peter Cousens tried to estimate the eventual number of v-CJD cases, based on the data from 14 cases.

Their worst-case prediction was 80,000, and their mid-range predictions between 1,000 and 10,000, spread over the next 30 years. The incubation period - between eating infected food and developing the disease - was estimated at 10 to 25 years.

Since then, no data has emerged to make those predictions look less accurate. Dr Graham Medley, an expert on CJD from Warwick University, said yesterday: "We still know very little about how long people carry CJD before they develop it, and it is therefore difficult to know what the scale of the threat is. If the incubation period is ten years, then we are in the middle of the epidemic. If it's 30 years, then we are only at the beginning. We have never seen this disease in humans and we simply do not know."