They 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. Plague too, rats dying all around, people passing into Hades. I have seen that myself in Baroda.
Ref : Letters on Yoga Vol. IV