sickness

Sickness is of the mind. It is insanity, mental illness, wrong-mindedness, the condition of the mind that believes in separation.
Physical sickness is made by us, by projecting onto the body our mental sickness. This projection is for a purpose: It reinforces the supposed “reality” of the ego. The Course mentions many ways in which sickness serves this purpose: a) The mind punishes the body to mitigate expected punishment from God and so usurp His perceived function (see T-5.V.5:4-9); b) the mind makes the body sick to prove that it is stronger than God Who would heal (see M-5.I.1-2); c) the mind punishes the body as a way of blaming the body for the “sins” the mind had the body act out (see T-18.VI.6:1); d) the mind punishes the body as a way of punishing itself for its supposed sins; e) the mind made the body out of the sickness of separation and projects this sickness onto the body (see T-19.I.7:7); f) the mind uses sickness to demonstrate that the mind is separate from goodness, health, and God (see T-28.V.1:1-5); g) the mind produces sickness to separate us from others (see W-pI.137.1-2); h) the mind makes the body sick to show how another has injured it (see T-27.I.3-4); i) the mind attacks the body because the body has failed to satisfy (see T-19.IV(B).11:6); j) the mind attacks the body to prove that the body is real (see T-29.II.8:1-3), that we are physical beings, not spiritual (see W-pI.136.7-8); k) the mind attacks the body because it suspects the body is not real enough to truly act out the mind’s fantasies (see T-18.VI.3:7); l) the mind makes the body sick to hide the fact that the real problem is in the mind, that the mind “attacks itself and wants to die” (W-pI.76.5:5).
The one true remedy for sickness is not physical medicine (though this can be appropriate to use; see magic), but the miracle, which heals all sickness regardless of form, thus proving that sickness is an illusion (see W-pI.140).