Organon §31

The power of natural diseases to make us sick depends on our susceptibility and on our exposure to them. 

2. The-partly psychical and partly physical-inimical potences in life on earth (which we call disease malignities) do not possess an absolute power to morbidly mistune the human condition. 73 We become diseased by them only when our organism is just exactly and sufficiently disposed and laid open to be assailed by the cause of disease that is present, and to be altered in its condition, mistuned and displaced into abnormal feelings and functions. Hence these inimical potences do not make everyone sick every time.  

73 When I call disease a tunement or mistunement of the human condition, I am far from wanting to give, thereby, a hyperphysical explanation about the inner nature of diseases generally, or of a single case of disease in particular. This expression is only meant to imply what diseases, as has been proven, are not and cannot be. They are not mechanical or chemical alterations of the material substance of the organism; they are not dependent on material disease matter. They are solely spirit-like, dynamic mistunements of life.