Cancer During our lives, cells in our bodies divide and replace themselves every second. Cancer starts when a cell is somehow changed so that it multiplies out of control. A tumor is a mass, composed of a cluster of such abnormal cells: Most cancers form tumors, but not all tumors are cancerous. Harmless, or noncancerous, tumors such as freckles and moles stop growing and do not spread to other parts of the body, and do …
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