Part of why cancer can be so hard to eliminate is because each cancer cell has the ability to self-replicate, so every single cell has to be removed.[113] Although the immune system is often able to control the very earliest phases of cancerous cell division, once a tumor does get established, it can develop resistance to the body’s immune cells. At that point, even if a treatment destroys most of the cancer cells, the survivors can start growing new tumors. A subpopulation known as cancer stem cells are especially likely to be these dangerous survivors.[114]
