Why diet and lifestyle won't cure your cancer, part 2

Two fundamental principles of evolution are random mutation and natural selection. An animal mutates in a way which gives it a slight advantage (bigger wings, etc) and if the environment favours an animal with this advantage, it is more likely to survive and have offspring which also have that advantage.
This, in a nutshell, is a perfect metaphor for cancer.

Your body is made of cells, and each cell contains a complete copy of your DNA. Genes are sections of your DNA which, when read by the cell, provide a set of instructions with which to make structures called 'proteins'. Proteins designed to do different things carry out all the functions of the cell, from making chemical reactions happen (enzymes), forming the wall of the cell (lipoproteins), to reproducing or committing suicide.

Remember those last two, we'll come back to them later.

The cells of your body replicate constantly at varying rates in order to keep up with how often those cells need replacing (red blood cells replicate slower than skin cells). Remember what we talked about with evolution, how mutations in a parent can be passed on to their offspring? That applies to every cell in your body. Mutations in DNA which occur in a cell are passed on to any cells made by that cell as it divides.

Every cell in your body can be thought of (for the purposes of this example, this isn't a literal definition) as a miniature animal. The only difference is that rather than each one competing with each other for survival, they all cooperate. Each individual cell might be able to get bigger and fatter on their own, but their cooperation means that they can live as an entire organism (an animal comprised of cells) and have a much better chance of long-term survival.

In order to facilitate this, they have all developed a way to stave off evolution because they don't want to gain an advantage over each other. When cells divide, there is ordinarily a mechanism in place by which the cell checks its DNA to make sure it hasn't received any mutations which give it an edge or disadvantage over its siblings. If it has mutated, the cell will attempt to fix the mutation before replicating. If the cell can't fix the mutation, it will commit suicide for the greater good of its siblings and the organism they make up.

It's a good system which usually works very well, 2 out of 3 people will go their entire lives never contracting cancer.
There is, however, one fundamental flaw: genes, made of DNA, are what provide the instructions for the mechanism by which cells look for and repair DNA.
Since genes themselves can mutate, the mechanism which fixes mutations can itself mutate. When this happens you get a cell which can't correctly fix mutations.

Now that it can't properly check itself for mutations, if the cell receives any further mutations which do give it an edge over its siblings, it can't fix them or commit suicide for the greater good.
Eventually, mutations occur which cause this cell to do things it normally wouldn't do. It starts replicating faster than it normally would, it starts growing bigger than it normally would, it might start invading areas it wouldn't normally go.

Sound familiar? It should, because at this point you have a cancer.