The discovery of the first chemical to prevent the death of brain tissue in a neurodegenerative disease has been hailed as the "turning point" in the fight against Alzheimer's disease.
More work is needed to develop a drug that could be taken by patients.
But scientists say a resulting medicine could treat Alzheimer's, Parkinson's, Huntington's and other diseases.
In tests on mice, the Medical Research Council showed all brain cell death from prion disease could be prevented.
Prof Roger Morris, from King's College London, said: "This finding, I suspect, will be judged by history as a turning point in the search for medicines to control and prevent Alzheimer's disease."
He told the BBC a cure for Alzheimer's was not imminent but: "I'm very excited, it's the first proof in any living animal that you can delay neurodegeneration.
The research team at the Medical Research Council Toxicology Unit, based at the University of Leicester, focused on the natural defence mechanisms built into brain cells.
When a virus hijacks a brain cell it leads to a build-up of viral proteins. Cells respond by shutting down nearly all protein production in order to halth the virus' spread.
However, many neurodegenerative diseases involve the production of faulty or "misfolded" proteins. These activate the same defences, but with more severe consequences.