The authors state the case that illness and disorder are part of an evolutionary reaction by the human body, and in some cases, they may be beneficial to the current physical or psychological state of the ill person. Examples include morning sickness as a control over the nutritional needs of the pregnant woman, and fever as a means to regulate body temperature to fight off infection. While the authors do not claim that medical treatment should be withheld, understanding the underlying causes of some medical illness or disorders may be as important as typical medical treatment.

The mystery of disease -- Evolution by natural selection -- Signs and symptoms of infectious disease -- An arms race without end -- Injury -- Toxins: new, old and everywhere -- Genes and diseases: defects, quirks and compromises -- Aging as the Fountain of Youth -- Legacies of evolutionary history -- Diseases of civilization -- Allergy -- Cancer -- Sex and reproduction -- Are mental disorders diseases? -- The evolution of medicine.