We all have tiny little cancers right now. Our bodies are constantly producing mutations that disable growth regulation, promote proliferation, change cell adhesion, etc.

It's just that our body is programmed to defeat these irregularities and disease states. The immune system is a major component, as it can recognize cell surface epitopes of a variety of types - mutated ones, or even suicide signals. Cells that detect that they have entered a disease state will often enter apoptosis or tell their neighbors about it.

We get cancer when cells accumulate so many mutations that they not only grow unchecked, but they also evade the body's natural forms of detection and clearance. And then they eventually hill climb into a state where they leave their original tissue and move about the body uncontrolled.

You evolved to defeat cancer. It's just a numbers game until one of your precancerous cells accumulates enough mutation to escape capture.