We have covered this topic many times here. Cancer may be aptly described as a defense mechanism on the part of groups of cells, but the idea of nature screwing up so badly as to have cancer as a defense mechanism of the body itself beggars belief. Why nature would have a defense mechanism that most often results in the death of the host is beyond me. It does make a terrific hook for selling a book though, the same as all the other universal causes and cures that have been put forth to sell books - cancer is a result of trauma, cancer is a fungus, cancer is caused by mysterious flukes . . .
If this debate continues it will soon find itself in the debate forum, which was created for debating cancer.