Throughout the human body cells grow and die at different rates. Each tissue has specific cellular requirements that define the growth and death of its cells. For example red blood cells are replaced frequently whereas brain cells slowly die with age and are not replaced. Cancer comes from a fundamental breakdown in the regulation of growth. These new growing cells reach a point when they are too abundant. When they have surpassed the body's need for new cells, they form large masses called tumors. Tumors are deadly when they become malignant. Malignant cells have the ability to invade and spread throughout other tissues of the body. They can also establish secondary growth areas in a process known as metastasis. Recruiting the blood supply and leaching nutrients from other areas of the body, tumors become a burden that ultimately weakens the host. A tumor's ability to grow and spread is rarely impeded by the immune system. This is because tumors have a unique ability to evade the immune system, making cancer a silent killer (Darnell, 1990).
Heinicke, G. (2012). Evaluation of Daily Oral Kinase Inhibitors in Pediatric SPNET Tumor Xenografts (Undergraduate honors thesis, University of Redlands). Retrieved from http://inspire.redlands.edu/cas_honors/11