"Sickness is a defense against the truth." - ACIM
Truth is divine perfect health. Truth is love. Truth is God.
When there is an unforgiving attitude, it produces sickness because it is an attack on yourself.
Sickness isn't a "thing". It doesn't have its own identity. It isn't a real thing. It's just a word for what it seems to look like when you are partly disconnected from or in denial of eternal life.
It is completely reversible because it is nothing more than an illusion. We simply need to return to a forgiving attitude by forgiving and undoing our unforgiving attitude.
Sickness must go. It does not have a power of its own. It is not a cause. It is only an effect, a side-effect of temporarily denying who you are.
It has no staying-power or causality. Sickness does not cause further sickness. Problems in the body do not cause sickness. Problems in the environment does not cause sickness. And sickness does not spread or increase on its own.
Sickness can only be an effect, or symptom. All notions of sickness being a 'thing' in its own right, that someone "has" a specific sickness, as though that sickness is a 'thing' that happens to them distinct from other things, that's not true.
And people don't "get sick" or "become sick". People choose not to be divinely healthy on some level.
Since sickness only seems to appear as an illusion when we are in denial of the truth, and since when we are not in denial of the truth sickness cannot exist, sickness does not really exist.
There is no "real sickness". Nobody is "really sick". They are experiencing illusions of sickness.
Sickness is nothing.
Sickness is not even the issue. It is just a symptoms of what it looks like when we defend against truth, which is a defense against ourselves, which is a self-attack. If we stop doing that, we will stop seeming to exhibit signs of sickness.
Only the truth is true.