Everyone in this world has a tendency to fall ill but some people have a better immune system than others. That makes some people less prone to infections. According to medical science, tuberculosis is a communicable and infectious disease but some people do not catch the infection even after staying with a T.B. patient.
According to Vedic astrology, diseases are denoted by sixth house, its ruler and the karaka of the sixth house, Saturn. Following the principles of bhavat bhaavam, being sixth from sixth, eleventh house would also denote diseases.
When a bhava, its ruler and the rashi gets aspected by malefic planets, the person gets the disease of the body part denoted by the particular planet. For example, when a horoscope shows an afflicted sixth house, virgo and its ruler Mercury, the person would suffer from hernia. Persons whose liver does not function properly should have an afflicted fifth house, its lord, Leo and Sun.
Sixth house shows the immune system - the inner strength to figth against diseases. Doctors, who spend a lot of time between patients treating them, must have a strong sixth house. If sixth house gets strong by having a malefic positioned there, it should be considered good for health, but it would not be considered for marriage. Benefits of a strong Saturn in sixth house, shows the ability to face trouble and fight diseases. On the other hand, a debilitated Saturn in sixth house, which would happen for Scorpio lagna, would give a cronic disease.
In case of Gemini ascendant, Mars becomes ruler of both sixth and eleventh house and hence becomes a double signifactor of disease. In this case, a person should be careful during the dasha of Mars.
According of Maharshi Parashara, Rahu in sixth house gives a longer life, however there are certain confusing and contradicting mantras given by various astrologers.