Cancer is the most complex and most difficult to treat illness. When damaged or infected normal cells are ordered to commit suicide by your immune system, they do it, because they are a part of you, the multicellular organism.
Cancer is when your own cells stop cooperating with you, they act as a separate organism and fight like hell to survive your body's attempts to destroy them. They use absolutely every possible mechanism they have to survive, subverting and distracting the immune system, creating new blood vessels to fuel it's own expansion, and sapping your own bodily resources to survive.
Other diseases, like bacterial infections, just want to live in you. The bacteria are just deer, when the hunter comes for them, they can't do much to stop him. Cancer is a mother tiger backed into a corner, protecting herself and her cubs. She's desperate and defensive, full of fury and the power to back it up. She lashes out with absolutely everything she has, even if it means risking her own life in the process.
This is just one reason why cancer is so difficult to treat, it's relentless and uses your own genome against you in every possible way it can.
