To whatever cause an illness may be due, material or mental, external or internal, it must, before it can affect the physical body, touch another layer of the being that surrounds and protects it. This subtler layer is called in different teachings by various names,—the etheric body, the nervous envelope. It is a subtle body and yet almost visible. In density something like the vibrations that you see around a very hot and steaming object, it emanates from the physical body and closely covers it. All communications with the exterior world are made through this medium, and it is this that must be invaded and penetrated first before the body can be affected. If this envelope is absolutely strong and intact, you can go into places infested with the worst of diseases, even plague and cholera, and remain quite immune. It is a perfect protection against all possible attacks of illness, so long as it is whole and entire, thoroughly consistent in its composition, its elements in faultless balance. This body is built up, on the one side, of a material basis, but rather of material conditions than of physical matter, on the other, of the vibrations of our psychological states. Peace and equanimity and confidence, faith in health, undisturbed repose and cheerfulness and bright gladness constitute this element in it and give it strength and substance. It is a very sensitive medium with facile and quick reactions; it readily takes in all kinds of suggestions and these can rapidly change and almost remould its condition. A bad suggestion acts very strongly upon it; a good suggestion operates in the contrary sense with the same force. Depression and discouragement have a very adverse effect; they cut out holes in it, as it were, in its very stuff, render it weak and unresisting and open to hostile attacks an easy passage.
[The Mother: CWM 3]