Medical Astrology helps to diagnose from signs, houses, and planets. The causes of ill-health can be known well before its presence in the human body through astrology. When bad planets influence the bad houses, then it leads to severe ill health.
The planets represent forces or energies which are present in the world and diseases are likewise represented by energies. Thus, a parallel can be drawn between them. Diseases are representative very much as manifestations of these planetary energies.
They shared the same qualities and attributes and thus, could be substituted one for another easily. This constitutes one of the fundamental foundations of medical astrology.