In the developed world, human life expectancy has more than doubled in the last two centuries and is still increasing. The trend may not be sustainable, but life expectancy is evidently continually improving. Our bodies intrinsically lack the resources to provide foolproof maintenance long after our reproductive years, but our individual capacities are notably variable because of the heterogeneity of our genetic legacies and our life experiences. Even before birth, nutritional stress, toxic substances, and viral infections can have effects on the development of the foetus that will shorten an adult life span. After birth, accumulation of somatic mutations – sometimes originating in poor nutrition or chronic inflammation – creates a risk of cancer and other diseases of late middle age and may advance the ageing process. However, although our bodies may be unable to repair all biochemical damage, we can, in principle, reduce some of the risk to health and life expectancy it imposes.

New health challenges originating in life experience still emerge. Breast cancer, allergic reactions, and obesity are three very different kinds of health threat that are becoming increasingly common in the developed world. Breast cancer was well known before the twentieth century, but the lifetime risk of developing such a malignancy in Britain and the United States (and probably in many other countries) has now reached one in eight.