Cancer is an inescapable consequence of life.
The building blocks of life are proteins such as DNA (deoxynucleic acid). These proteins control mammalian life by producing the structure of the body, determining how they grow and work and how they are controlled and repaired.
To perform life functions, cells need to reproduce protein, for example to replace a damaged cell. This is achieved by having DNA, (a complex paired strand of amino acid coded building blocks), separate into two and regenerate a mirror image. Since this involves thousands if not millions of chemical interactions, occasionally it will go wrong.
It makes sense therefore, if errors are going to appear it will be in cells reproducing often, (such as skin or bowel surface), and will be more likely with time as we get older. Cancer is an erroneous cellular reproduction leading to a defective cell line (clone), which is dysfunctional, particularly as regards limitation on growth.
Since these defective clones are part of the individual, they are not recognised as foreign and thus are not normally rejected. Thus the growth is often not painful unless it involves other sensate tissues, and tends to be progressive.
Several environmental factors increase the production of cancers in humans. Classics are sunlight to the skin and cigarette smoke to many organ systems. Neither of these influence the incidence of Breast or Endocrine cancers.
Cancers are variable and as complex as life and an individual, each is unique. Having said that many behave in predictable fashion, and although some outcomes are less than favourable, today most are effectively treatable.