The cause is that DNA mutations makes the cells go "feral", and turn into a new mono-cellular species that invades the body. DNA can be damaged in several ways,

The cause is that DNA mutations makes the cells go "feral", and turn into a new mono-cellular species that invades the body. DNA can be damaged in several ways, by UV rays or mutagenic chemicals, but it can also happen during cell-division. Any type of damage, even micro damage, the body sustains means that cells have to start dividing to repair it. But cells also continuously divide to replace old ones even without damage. So all in all, it's just a matter of time until you get unlucky, something goes wrong and you have a cancer.