What is the difficulty [in understanding how the subtle forces of illness attack the body using bacilli and viruses for their purpose]? You are like the scientists who say or used to say that there is no such thing as mind or thought independent of the physical brain. Mind and thought are only names for brain quiverings. Or that there is no such thing as vital Force because all the movements of life depend upon chemicals, glands and what not. These things and the germs also are only a minor physical instrumentation for something supraphysical.
They [the forces of illness] first weaken or break through the nervous envelope, the aura. If that is strong and whole, a thousand million germs will not be able to do anything to you. The envelope pierced, they attack the subconscient mind in the body, sometimes also the vital mind or mind proper—prepare the illness by fear or thought of illness. The doctors themselves said that in influenza or cholera in the Far East 90 per cent got ill through fear. Nothing to take away the resistance like fear.
But still the subconscient is the main thing. If the contrary Force is strong in the body, one can move in the midst of plague and cholera and never get contaminated.
Ref: Letters on Yoga – IV