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How does cancer happen?

Cancer begins in your cells when the genetic material, or DNA, is damaged or changed. DNA tells your body how and when to make new cells. Normally, your body forms new cells as you need them, replacing old cells that die or become damage. When the DNA is damaged or changed, cells might not die when they should and new cells might form when the body doesn’t need them. These extra cells can form a mass called a tumor. Some tumors are cancerous, or malignant. Cells from malignant tumors can invade nearby tissues. They can also break away and spread, or metastasize, to other parts of the body through the blood and lymph systems.