WHAT IS CANCER? Cancer is when abnormal cells divide in an uncontrolled way. Cancer starts when gene changes make one cell, or a few cells, begin to grow and multiply too much. This may cause a growth called a tumour. A primary tumour is the name for where a cancer starts. Cancer can sometimes spread to other parts of the body – this is called a secondary tumour or a metastasis. Cancer and its treatments can affect body systems, such as the blood circulation, lymphatic and immune systems, and the hormone system.