One of the fundamental transformations in twentieth century public health and medicine has been the widespread acceptance of a new concept of the causes of chronic and degenerative disease. This is the lifestyle theory, which holds that an individual's state of health is affected by specific aspects of the manner of living of that individual. In the same way that an individual suffering from a disease must follow a prescribed regimen to recover from that disease, a healthy individual must engage in continuous activities that are an integral part of daily life in order to maintain health. According to the theory, the behaviors involved in healthy lifestyles can increase or decrease the probability that an individual will develop particular diseases.

The idea that the maintenance of health requires continuous personal care and attention is an ancient one and a basic precept of the Hippocratic writings. Until the twentieth century, however, the behaviors prescribed for maintaining health were vague, ill–defined, and usually in the form of aphorisms: moderation in all things; early to bed and early to rise; an ounce of prevention is worth a pound of cure. The recommendations were rarely enlightening because they were neither internally consistent nor based on scientific investigations. Most healthy persons did not believe that the maintenance of health required a specific lifestyle and lived without much concern about the effects of their daily actions on future health. Furthermore, many illnesses were considered an unavoidable part of the normal pattern of life and therefore beyond human control: the infectious diseases of infancy and childhood; women's deaths, diseases, and disabilities resulting from childbearing and housework; workers’ deaths, diseases, and disabilities produced by hazardous, unhealthy, and arduous working conditions; and the infirmities produced by aging. Even if the diseases or deaths were preventable, the great majority of people lacked the financial and other resources to make the necessary changes in their lifestyles to avoid them.