According to the WHO, Cancer is a generic term for a large group of diseases that can affect any part of the body. One defining feature of cancer is the rapid creation of abnormal cells that grow beyond their usual boundaries, and which can then invade adjoining parts of the body and spread to other organs. This process is referred to as metastasis. Metastases are the major cause of death from cancer. Cancer is classified either according to the kind of fluid or tissue from which they originate or according to the location in the body where the cancer first developed. Leukemia is a type of cancer which affects the blood cells. The body is made up of many different types of cells, with leukemia; it is white blood cells that are affected. In leukemia white blood cells do not grow properly, remain within the bone marrow and continue to reproduce in an uncontrolled way. These cells fill up the bone marrow and prevent it from making healthy white blood cells. This means the body is less able to fight with infections.