Cancer is defined as the abnormal or unrestrained growth of cells that spread to surrounding tissue, causing blockages, destroying nerves or eroding bone.
The change from a normal cell into a cancerous cell happens over time, as defects in the cell's DNA maintenance system build up.
The cell's DNA maintenance system controls cell proliferation, the cell's relationship with neighbouring cells, and its capacity to escape the immune system's surveillance.
Once cancerous cells have developed, they can also spread through blood vessels and the lymphatic system to form secondary tumours - this is a process called metastasis.
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Source: Al Jazeera