From "Sickness is a defense against the truth", Lesson 136:
"No one can heal unless he understands what purpose sickness seems to serve. For then he understands as well its purpose has no meaning. Being causeless and without a meaningful intent of any kind, it cannot be at all. When this is seen, healing is automatic. It dispels this meaningless illusion by the same approach that carries all of them to truth, and merely leaves them there to disappear."
Here Jesus tells us simply that if we can clearly recognize a sickness for what it IS - an illusion, it will disappear automatically.
When we look at sickness we think it has a variety of causes. None of them are true. We also think it has a valuable purpose. The purpose is supposedly to defend our ego identity against truth. That's because we're mistaken about the truth and think that it will destroy us, so we "naturally" defend against it. Defense (denial) manifests reasons to believe that illusions are more real than God.
It is because we have a multitude of beliefs that tell us that sickness is real, that we have the sickness at all. It is sustained by those beliefs and created by them. It seems the sickness has no purpose and is a victimization, but really it does have a secret purpose. Its purpose was to block reality by demonstrating that illusions are real. Therefore we 'manifest' illusions that seem real, to try to prove that this is true. Manifesting illusions and making them real invents sickness.
We need to become willing to see instead that the purpose of sickness was to block the truth, not to make something true. It's purpose wasn't to affirm reality, it was to lend reality to something that is unreal. It's purpose wasn't to confirm how real the body is, but to deny how real the truth is. And finally we need to become able and willing to RECOGNIZE that sickness is an illusion, NOT a reality.
This is the defining principle. Sickness is the illusion that illusions are real. It is a denial of the truth - an attempt to hide the truth from awareness. This denial of truth causes sickness because it is your attempt to make God sick and dead. Your attempt to destroy God destroys you. Your belief that God (reality) is sick, produces sickness in your reality.
Your challenge in undoing sickness is to shift your sense of reality away from the sickness and place it on the real truth - God. If the sickness is true and real, God cannot be. And because your sickness is a choice to believe illusions are more real than reality, they are choices to believe that God is not real. They therefore block God, which is the giver of Life, and blocking life manifests as sickness (and death).
To undo sickness you must be willing to admit and recognize that sickness is not REAL, it is an illusion. It can't be real because of the ways in which God is real. God has to be more real than the sickness otherwise you will keep the sickness and continue to believe in its reality. Your belief in its reality is what puts it into your experience, because you are mis-creating.
You need to un-believe that the sickness is real in all its respects, in every way. There is nothing about sickness that can be "true" for you. Otherwise you will value it and keep it. When you can see that it LITERALLY is NOTHING, i.e. its an illusion that isn't real at all, you will cause it to disappear, because you will cease to GIVE it reality.
That's a miracle, an acknowledgement of God's truth and its equal meaningfulness to everyone.