Cancer is a disease in which the control of growth is lost in one or more cells leading to a solid mass of cells known as a tumour. A growing (primary) tumour will often become life-threatening by obstructing vessels and/or organs. However, death is most often caused by spread of the primary tumour to numerous other sites in the body which makes surgical intervention impossible. Other types of cancers such as leukaemia involve a build-up of large numbers of cells in the blood.