Transformation of normal cells into cancer cells in the body, is not a rare event. But the body’s defence or immunologic cells—lymphocytes and macrophages—at once recognise these abnormal cells and kill them before they get divided and subdivided into many more.
In a person exposed to carcinogenic or cancer causing agents, this transformation of normal cells into cancer cells occurs more often and some of the cancer cells do not get killed in time and so are able to set up a cancer.
Moreover, an immune deficient person exposed to carcinogenic agents the normal surveillance provided by the defence immunologic cells is weakened, so that not all cancer cells get killed and those which remain, are in a position to set up the disease.