So where does it come from?

Population studies begun forty to fifty years ago show that when people migrate from one country to another, they acquire the cancer rate of the country to which they move, despite the fact their genes remain the same. This strongly indicates that at least 80 percent to 90 percent—and probably closer to 97 percent to 98 percent—of all cancers are related to diet and lifestyle, not to genes.— Whole: Rethinking the Science of Nutrition, page 129

<idle musing>
I remember when I was an undergrad at the University of Wisconsin; I was hitch-hiking to school and a couple of grad students picked me up who were working on this kind of stuff (way back in the 1970s); they were discussing the results of their work with each other. The one guy said to the other, "I'm convinced that cancer is man-made." That's stuck with me—obviously, if I can still remember it 40 years later!
</idle musing>