Why is cancer difficult to cure?
Spreads very fast, Undetected by the immune system
Cancer is caused due to mutated cells. When a person is exposed to radiation or chemicals, some of the cells' DNA gets mutated and this causes the cells to stop responding to the signals. Due to this the cell undergoes uncontrolled mitosis and forms tumour.
During a chemo or some treatment most of these cells are killed but some manage to remain unseen or hidden and they again start to replicate. And soon new tumours are formed due to metastasis and the person again suffers from cancer.