When looking upon the law of Karma you must think of the future, for with everyone of our actions we enter into our account book an item which will bear fruit. […] Many things become clear to us through an insight into this law. In the first place, we can accurately prove the connection between the individual bodily development and earlier lives. A life full of love prepares for the next life a course of development whereby the human being preserves his youth for a long time; a premature ageing is on the other hand caused by much antipathy during the past life. In the second place: A particularly selfish sense of grasping and hoarding things produces in the next life a disposition to infectious diseases. In the third place, it is of special interest that pains, and particularly certain illnesses through which we pass, produce a beautiful body in our next life. This insight enables us to bear many an illness more easily.
Source: Rudolf Steiner – GA 100 – Theosophy and Rosicrucianism: Lecture VII: The Law of Karma – Kassel, 22nd June 1907