As we have discussed in earlier chapters (Chapter 5 and 6), it is likely that less than 5% of human cancers are directly caused by genetic abnormalities. Genetic predisposition, on the other hand, plays a much greater role in the development of human cancer, but also requires environmental input. Thus, one may suggest that up to 90% of human cancer is caused by environmental factors. Figure 12.26 is a diagram of the various environmental causes of hu- man cancer as originally proposed for the United States population by Doll and Peto (1981). Although this chart is more than two decades old, there is substantial evidence to support these proportional contributions. While the proportion of cancer due to infection is higher than de- picted in the figure (see above), this change would not dramatically affect the overall distribu- tion of causations. As we have noted in this and the previous chapter, environmental factors contributing to cancer causation are, at least theoretically if not practically, remediable. In fact, at least two-thirds of human cancer could be prevented by appropriate actions, individual and societal. Although cancer therapy, as we shall see later in the text, has made enormous strides during the last few decades, it is likely that our knowledge of the significant environ- mental factors in the causation of human cancer will lead to extensive programs of prevention in the near future.