The human body is made up of billions of cells. Cells are so small that they can only be seen under a microscope. Single cells must divide many millions of times to become mature living things. In the process, many different types of cells are developed to form the special parts which make up the whole working body. Diagram All through life, many cells are constantly wearing out, dying off and being replaced by new ones.
Cancer is the name given to a group of diseases which start in the cells of the body.
It may start in one cell which divides in an uncontrolled way, affecting the normal functioning of the body. Sometimes a group of cells change, grow differently and more quickly than the cells around them. These may become cancer cells. These cells grow into groups where other normal cells should be growing. This is called metastasis, and can start a secondary cancer.