What really causes cancer?
Cancer is caused by the mutations in cells.

Cancer exists because of the failure of cell cycle checks which could actually allow cells to survive with erratic genomes. However, that alone does not cause cancer. 

Cancer is the uncontrolled proliferation of cells. That only happens when cells have accumulated enough errors during cell division and each cell division goes undetected to the point that cells refuse to commit apoptosis; they are no longer receptive to the body's control mechanisms. 

Cancer cells are also malignant. That means that cancer cells cannot be called cancer cells until they are invasive. That means a few cancerous cells cannot be caused cancer because they don't take nutrients from the body . Also, tumour  suppressor  genes can also cause cancer when they turn proto-oncogenes into oncogenes.