"Cancer" is the general name for a group of more than 100 diseases that affect the body at a cellular level. Although there are many kinds of cancer, they all start because abnormal cells divide and grow out of control.
Normal body cells grow, divide and die and are replaced by new cells. Cancer cell growth is different from that of normal cells. Instead of dying, cancer cells continue to grow and form new, abnormal cells. Cancer cells can also invade (grow into) other tissues and organs, something that normal cells do not do. When cancer spreads in this way, the condition is called metastasis.
Why does cancer happen?
Cells become cancerous or malignant because of damage to their DNA—this is called a cell mutation. In a normal cell, when DNA gets damaged the cell either repairs the damage or the cell dies. In cancer cells, the damaged DNA is not repaired, but the cell doesn't die like it should. Instead, this cell goes on making new cancer cells.
Cancer is a disease with a long latency period. This means that it may take years from the time of the first mutation to when signs of the disease appear. This is why cancer screening is so important: to detect cancer earlier, before we can see or feel it.
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Source
American Cancer Society. What is cancer? Accessed July 31, 2011.