THE thorniest problem facing the health-care profession is how to strike the right balance between promoting health and curing illness. As is routinely pointed out, prevention is better than cureand cheaper too. But the forces ranged against this benign cliché are formidable. The sick require immediate treatment. The medical profession values surgeons more than dieticians. And most of us are greedy and short-sighted: why forgo the instant ecstasy of a Mars bar, or the joy of unprotected sex, when the rewards of restraint are so distant?