Normal cells become cancerous when changes in their DNA force them to grow and form new, abnormal cells. Rather than dying in an orderly way, cancerous cells grow out of control and invade or grow into other tissues, according to the American Cancer Society.
A cell becomes abnormal when its DNA, the source of replication instructions, becomes damaged. Unlike normal cells, cancerous cells replicate infinitely, do not know when to die, and may break away and grow in other parts of the body. If abnormal cells are few, the immune system keeps them under control, and they remain harmless. When these cells grow and divide uncontrollably, they form growths and lumps commonly known as tumors. Unlike tumors that aren't malignant, cancerous tumors spread and destroy the surrounding tissue and cause additional tumors to develop, explains the U.S. National Library of Medicine.
People can inherit DNA abnormalities, but most faults in the DNA arise from cell replication processes and environmental factors such as cigarette smoke. However, the exact causes of these faults remain unknown. These faults prevent damaged cells from repairing themselves or dying. When cancerous cells get into the bloodstream or lymph vessels, they spread and crowd out normal tissue, reports the American Cancer Society.