Cancer is a disease of the cells in our bodies. There are millions of them, but the disease usually starts with just one. What appears to happen is that the genes which control how the cell divides and multiply are damaged in some way. The effect of this is that cell division goes out of control as the life cycle of the damaged cell is altered. It continues to divide, but its daughter cells are damaged too. Eventually the presence of so many damaged cells creates a tumour.
We are all at risk of cancer and several factors are known to increase that risk such as smoking, radiation, unhealthy diet, chemicals, age, infection, a poor immune system, and individual genetic susceptibility. It is likely that the original cause of a cancer is a combination of these factors as not everyone who smokes, or sunbathes excessively without protection, or who has a poor diet will develop cancer.