Scientists expect that new knowledge of the relationship between genes, proteins and disease will bring new cures. But before it does, it is bringing new tests for the likelihood of people falling ill - promising a world where, increasingly, people who feel perfectly well are encouraged to take drugs just in case.

If an anti-Alzheimer's drug was discovered, an Alzheimer's test like the one just successfully trialled in mice could lead to millions of healthy people taking drugs for years against the possibility of their developing the disease.

Earlier this month Cancer Research UK said it would support offering the powerful anti-cancer drug tamoxifen to healthy women if it could be shown they were genetically predisposed to breast cancer.

It is already routine to prescribe drugs, as well as offer lifestyle advice, to people whose high blood pressure or stubbornly high cholesterol levels make them prone to heart attacks.