This may be a stupid question but
It is caused my radiation, mutagen substances, or naturally occurring cell mutation.
Cancer is cause by a mutated cell that eventually multiplies and become life threatening. They way we are practicing our life style contribute to cancer. For example, we can get skin cancer by tanning, over sun exposure and burning the skin (tanning). We can get lung cancer by smoking or having an respiratory diesease for so long: Empzyma, Bronchitis, TB.
Our Genes and Genetics play a role in cancer. If you have Lukemia that runs in your family this disease may appear in you. Not only that you need to be careful about what you do in life that contributes to eating, or doing risky things that involves risky situations and behaviors.
Cancer, can come from anything food water the air it's all the chemicals used to purify Yeah Right!
Most oncologist believe cancer is a multi-step process involving mutations to two sets of genes.
The first set are called oncogenes. A normal gene with a cell division function (proto-oncogene) may be mutated to become an oncogene. An oncogene can be introduced by some viruses also. Oncogenes produce abnormal protein instructions which may lead to uncontrolled cell growth.
The second set of mutated genes are tumor suppressor genes. These genes prevent cancer from starting by either repairing DNA damage or committing suicide (apoptosis) if the damage is too extensive.