Good habits will produce good health; bad ones will create a tendency to some specific illness in the next life. A strong determination to rid oneself of a bad habit will work down into the physical body and produce a tendency to good health. How a disposition to infectious diseases arises in the physical body has been particularly well observed. Whether we actually get a disease will depend on what we do; but whether we are specially liable to contract it is the result of the inclinations we had in a previous life. Infectious diseases, strangely enough, can be traced back to a highly developed selfish acquisitiveness in a previous life.
Source: Rudolf Steiner – GA 95 – At the Gates of Spiritual Science – Lecture VII: Workings of the Law of Karma in Human Life – Stuttgart, 28th August 1906
Translated by E.H. Goddard & Charles Davy