If a man has bad habits and bad characteristics and does nothing to get rid of them, this will appear in the next life in the physical body as a disposition, a predisposition to illness. Strange as this may sound the disposition towards certain illnesses, particularly infectious ones, depends on the bad habits of a preceding life. This insight therefore enables us to prepare health or illness for our next life. If we conquer a bad habit, we become healthy and immune against infections in our next life. Thus we can prepare health for our next life. By endeavouring to foster only noble qualities, we can prepare a healthy body for our next incarnation.
Source: Rudolf Steiner – GA 100 – Theosophy and Rosicrucianism: VII. THE LAW OF KARMA – Kassel, June 22, 1907