Primary prevention, broadly defined as the protection of health by personal and community-wide efforts (1), consists of measures aimed at preventing the inception of a pathologic process or the occurrence of a disease, in contrast with secondary prevention that consists of measures for the early detection and prompt intervention on a clinically asymptomatic disease. Referring to cancer, primary prevention mainly involves the avoidance or drastic reduction of exposure to carcinogenic risk factors.

Primary prevention of cancer has evolved through the ages, in close relation to the evolution in our understanding and interpretation of cancer etiology. In ancient times etiology and prevention of cancer were entirely included within the concept of this feared disease being equivalent to divine punishment. Recognition of one's sins, repentance, and pious obedient behavior opened the only available path to God's forgiveness and provided the only possible protection from cancer. Traces of such belief are still with us, not only as remainders of old superstitions but also as components of today's attribution of a prominent role in the origin of disease to individual behaviors, habits, and lifestyles. Individual behavior and lifestyle certainly play important roles, but today's trend appears to be that individuals are considered not only responsible for but also guilty of causing their disease, a situation very close to suffering the curse of God.

The spectrum of diseases considered to be self-inflicted is wide, ranging from those related to the use of tobacco to excessive alcohol consumption and from overnutrition to lack of exercise. However, the assumption that all behavioral choices are free choices does not reflect the actual situation. Apart from the obvious fact that certain individuals may be genetically predisposed to some conditions, the commonly used term "lifestyle" does not distinguish the various causes of habitual behavior, such as smoking, alcohol drinking, unhealthy dietary habits, and lack of physical exercise. Among these causes, social pressures and ubiquitous advertising play an essential role (2). Only a minority of inner-directed, strong-willed people can oppose or resist such pressures and make fully autonomous choices. Moreover individuals cannot really choose the socioeconomic situation in which to be born or their genetic background, and most workers cannot choose to avoid working in hazardous industries or occupations.

The only time that cancer was considered to be transmissible was between the seventeenth and eighteenth centuries. Special hospitals were built in certain European countries to isolate patients with cancer, almost in the same way as was done for patients with leprosy (3). Relatively soon, however, the conviction that cancer was not contagious prevailed again. It was strengthened during the glorious period of microbiology by the inability to identify a germ or bacterium that could be related to cancer. This may perhaps partly explain the scant attention paid to the experiments of Peyton Rous (4) at the beginning of the twentieth century on the role of viruses in the origin of tumors. In the first half of the twentieth century, the hypothesis of a viral origin for human cancer gained the support of great scientists such as Oberling in France and Zilber in the former Soviet Union (5). It became a favorite hypothesis at the time of President Nixon's war against cancer in the early 1970s, and has recently returned to the stage with renewed strength (6).

Besides the hypothesis of infectious (parasitic or viral) etiology, other early prevailing speculations on the origins of cancer have been the cell irritation theory, proposed by Virchow, and the embryonic "cell rest" theory of Conheim, who proposed that tumors arise from embryonic cells that fail to mature and persist in the tissues (3,7-9). Of more than anecdotal interest may be that the 1905 report of the Huntington Cancer Research Fund, noting the unsuccessful attempts made to identify the causes of cancer, stated that "It has therefore seemed advisable . …