An expert in the human form of "mad cow disease", variant CJD, has warned it could become endemic in the UK unless blood screening is developed quickly.
Variant CJD can be passed on in blood transfusions
Professor James Ironside, from the National CJD Surveillance Unit in Edinburgh, said the disease was still a major threat, despite a drop in deaths.
He is to warn an international conference in the city that the risk to the blood supply continues.
There have been 161 vCJD deaths since the disease emerged in early 1990.
Speaking in a lecture ahead of Prion2007, the leading scientific conference on CJD and similar diseases, Prof Ironside is to challenge the popular perception that the vCJD outbreak of the 1990s was an isolated threat to humans.
He said he hopes to clarify the risk of human-to-human transmission posed by the thousands of people who may be infected without showing any symptoms.
Prof Ironside, who has significantly advanced knowledge of the neurodegenerative disease, said: "Although the number of BSE and vCJD cases is dropping, we ignore these diseases at our peril.
"We know that a significant number of people could be infected with vCJD without showing symptoms.
"However, we do not know how many people may be affected, and there is no cure or treatment."
He added that until a rapid screening test is developed, unknowing carriers pose a great risk of infecting others through donating blood or having surgical operations.
Blood transfusion centres have tried to reduce the danger of disease being passed on by removing the more risky white blood cells and banning anyone who had already received a transfusion from donating.
The public lecture, organised by the University of Edinburgh, will be held at 1800 BST in the Assembly Hall.
It will also include a presentation from Professor Marc Turner, a clinical director of the Scottish National Blood Transfusion Service.