It is generally the Sun-sign, the Moon-sign and the Ascendant of the person which establishes in a person, where he would have trouble. For eg. a person who is born under Aries is likely to face trouble with his head such as head-aches etc.
Apart from body parts, the signs are also associated with the various physiological systems, eg. Cancer is associated with digestion.
As each sign is ruled by a particular planet, these also form a correlation with the body, like the Mars is associated with fever.
In the horoscope, it is the sixth house that pertains to health matters. There are lot of considerations to be made, like the aspects as also the correlations among the various planets involved. This science can be quite helpful in not only diagnosing a disease and predicting the future illness, but also in aiding a remedy.