Jesus refers in the course to sickness as "a magical solution". It's an attempt to solve a problem. The only problem is, this solution is mistaken, because it actually does something opposite to what the person thinks it will do.
If you are confused about what the truth is, you perceive the truth as something false or evil. You even will feel threatened by it. Then you will seem to defend yourself against it. And so you will erect defenses to push the truth away. The trouble is, truth life and health. So when you push it away, even on grounds that you think it is not truth, you are pushing life and health away. And this results in sickness. Even if you believe you have pushed sickness away.
"Sickness is a (mistaken) defense against the truth."
The person then ends up sick, using sickness as a defense, to try to protect themselves, to save their life, from the truth. But the truth is life, and what they're really doing is trying to save themselves from life. And this is why they've induced illness by mistake, thinking that they're inducing life. They've mistaken life for death, and have chosen death by mistake.
"Sickness is but a little death."
All this means is that sickness is a MISTAKE. It's what someone produces when they are MISTAKEN. They come to a wrong conclusion, based on a false belief or a wrong perception, and then seek to erect defenses that they don't even need, and try to protect themselves from something beneficial, believing that they'll be better off. There is no SIN involved here, it's just a matter of being very mistaken. A choice made on a false premise.
The form of sickness, even physical death, becomes nothing more than the outcome of someone's mind that is confused. It's the result of a creative person, a person who is powerful and making choices, who happens to just be making unwise choices, because they don't realize they are mistaken. They are confused about what is true, they are not entirely sane, and so they THINK they're making a totally sound choice and acting on it, when in fact they don't really recognize what they're doing wrong.
So now you've got a world filled with powerful creative minds, who are constantly choosing and creating, but who are confused. And so they keep creating mistaken stuff, stuff which hurts them, stuff which produces suffering, stuff which makes their bodies sick, not because they're inherently sinful or guilty or deserve it, but simply because they are mistaken about what things are. They don't recognize what is really true, and have made false conclusions, and now are choosing things which they think are one thing, but are another.
To be sick therefore is nothing more than a mistake in the mind. It just means the mind isn't really recognizing what the truth is, and has mistaken the truth for something false, and mistaken something false for something true. And now they're convinced that they are in fact pursuing the truth, all the time, except their navigation system is off because their truth-detector is faulty. And so they head towards death, thinking it is life. And hell is confused with heaven.
Everyone always protects what they believe is true. What is true is your treasure. What is true defines your identity. It's close to your sense of self. Of life. Of what you think is holy and pure and good and worth having. Truth is what you accept and allow and want. But if your idea of truth is faulty, if you are confused and mistaken, you will still have this attitude about your truth, except it won't really BE the truth in the big picture. But you don't know that, because you have mistaken something false for something true.
The simple confusion of what is true and false is the entire basis for people producing ALL of their own suffering. It's merely a mistaken choice. It's powerful creators mis-creating by mistake because they have not mastered their own creative power. And result is a world if conflict and suffering which the people are creating by mistake, being unmasterful of their abilities.
It's chosen by mistake, or in error. A wrong conclusion was arrived at. A faulty response was activated. Someone thought that a "magical solution" would work, when in fact it produced the opposite effect. And everyone who has chosen sickness thinks that they have chosen continuity and life. Even those who have chosen death believe they have chosen the preservation of themselves.
All suffering is just a mistake. Mistakes call for correction. They don't need judgement or condemnation or punishment. No-one needs to be kept suffering. No-one deserves to linger in torment. Or to be destroyed or killed. Even the most heinously evil-seeming person is merely a person who is very confused about what is true and false. They are just insane, lost, alone, frightened, uncertain, out of touch with reality, and making poor choices. They just don't know what the truth is.
This is why ACIM is constantly suggesting that we simply "choose once again". That we have made a wrong decision somewhere along the lines. A mistaken choice. Which has produced unfortunate effects, symptoms, conditions. And now we are suffering because we chose something on faulty grounds, without proper justification. And now if we don't want to keep suffering, we need to have a little willingness to be open to being mistaken, to recognizing that our idea of truth is faulty. To recognize that we just made a mistake and need correction.
Then we can be willing to not only open up to a greater truth, to accept atonement, but also to allow the Holy Spirit to undo the consequences that we came up with. The unfortunate "mis-creations", the side-effects of unwise decisions, the suffering we induced for ourselves due to being a confused creator. All we need is to become un-mistaken. To recognize our mistakes and receive correction. So that instead of being mistaken, we are correct. And when you are correct, you can cause nothing but immortality and happiness, based on truth.
"God's teachers choose to look on dreams a while. It is a conscious choice. For they have learned that all choices are made consciously, with full awareness of their consequences."
"Say to yourselves the following, as sincerely as you can, remembering that the Holy Spirit will respond fully to your slightest invitation:
I must have decided wrongly because I am NOT at peace.
I made the decision myself, but I can also decide otherwise.
I WILL to decide otherwise, because I WANT to be at peace.
I do NOT feel guilty, because the Holy Spirit will undo ALL the consequences of my wrong decision IF I WILL LET HIM.
I WILL to let Him by allowing Him to decide for God for me."