Think of it like fighting a person with a sword and a shield. You know that all together you cannot overcome him. So, you decide to feint (or fake an attack) at his sword arm. This causes him to raise his shield to deflect the incoming blow. Now that his guard is up, you can begin to hammer away at the shield, slowly lowering his durability. Eventually, he will become tired and lower the shield. This is where you strike the killing blow. Cancer is the same way. It attacks the blood then goes for the defense. When the defense is low, it attacks the entire body, piece by piece. It slowly spreads throughout the body and begins taking down whichever organ its designated to first.
