There is a powerful subconscious reaction that influences a disturbingly wide range of our daily behaviour - our eating habits, our relationships, our values. The very same reaction that makes us draw back, lip curled, when we step on dog dirt is also constantly at play in our lives. It is called disgust. Compared with love and fear, it has been given little attention. Yet a raft of studies show it influences what we wear, what we eat, what products we buy, who we desire, and how we vote. It underlies our attitudes to those perceived to be outside the norm: be it overweight, disfigured, or homosexual. It even guides our moral judgement. How and why did such a powerful emotion evolve? Why do people in widely differing cultures all exhibit disgust at the same things? Valerie Curtis presents a powerful theory based on recent experiments: that its origins lie in the avoidance of parasites. But in humans, with our complex social lives, it seems that the disgust response has spread much wider than its original health-promoting role. Understanding its evolutionary origins helps us both to counterbalance its harmful manifestations, such as sexism and xenophobia, and exploit it for good: Curtis is widely known for her work in promoting hygiene and health care programmes worldwide - work in which the harnessing of the potent disgust response pays great dividends.