Each planet & sign represents a certain area of your body as well as functions, and diseases can be linked to that. In the chapter Sings you can find more information to what body parts are ruled by which sign/planet. Problems to a certain area of your body are related to an affliction towards the planet that is ruling the specific body part.
An affliction occurs when a certain planet is badly placed within a birthchart such as Venus placed in Aries, and when harsh aspects are made to that planet (by squares or oppositions), or when a planets is conjunct with a malefic planet such as Venus conjunct Saturn.

This can either be interpreted in a Natal chart, Solar Return chart, Transit chart.
It has more effect when a certain planet is already badly placed and aspected in the Natal chart + harsh aspects in transit.

Meaning of the Planets
An affliction occurs when a planet is in aspect with Saturn, Mars, Northnode or Southnode. Or when a planet is in exchange with one of these, this is considered the strongest affliction. More info regards affliction :

1) Aspect by Mars, Saturn, Northnode, Southnode
2) House/ Planet in aspect with 6th,8th or 12th ruler
3) Planet is afflicted in the houses 6, 8 or 12
4) Combust planet (conjunct Sun)
5) A planet positioned between two malefics

Sun :
Bones, heart, stomach, right eye, skin, head, joints
Afflicted: high fevers, mental illness, problems to the bodyparts the Sun rules over.

Moon :
Heart, breasts, left eye, lungs, brain, blood, body fluids
Afflicted: sleeplessness, lack of intelligence, astma, blood related problems, problems related to diabetes, menstruation, appendix, lung disorders, coughing, vomiting

Mars :
Energy of a person, neck, blood, female organs, genitals, red blood cells
Afflicted: Brain disorders, injuries, toxicity, painful eyes, itching, blood clotting, female genital disease, weak bones, tumor, piles, blisters, knee problems

Mercury :
Galbladder, nerves, lungs, hair, chest, tongue, arms, face
Afflicted: muscle and chest problems, typhoid, madness, paralysis, seizure, cholera, dizziness

Venus :
Face, urine, semen, throat, glands
Afflicted: eye disorders, genital disease, throat issues, fever, gonorrhea, syphilis, rheumatism, blood loss

Jupiter :
Fat, brain, liver, kidneys, lungs, ears, tongue, memory
Afflicted:  diabetes, problems with ears and tongue, memory and pancreas

Saturn :
Legs, joint bones, muscles, teeth, organs, skin, hair, knee
Afflicted: physical weakness, muscle weakness, stomach pain, organ injury, disease to the skin or feet, joint pain, blindness, paralysis, deafness

Northnode :
Legs, breathing, lungs
Afflicted: breathing problems, cataracts, blisters, cancer

Southnode :
Afflicted: fever, problems with lungs, insects in the intestines, ear problems, eye disease, stomach pain, physical weakness, brain disorders

Meaning of the Signs 

Aries : Head, brains, inflammation, fever, face
Taurus : Throat, endurance, thyroid gland, gums, middle ear, lips
Gemini : Hands, nervous system, harms, shoulders, lungs
Cancer : Breasts, chest, stomach, mucous membranes, uterus, ribs
Leo : Heart, energy, vitality, spine, middle back, muscular portion of the heart
Virgo : Small intestines, pancreas, duodenum, enzyme production, fingers
Libra : Waist, lower back, skin, glandular vascular part of the kidney
Scorpio : Genitals, colon, bladder, large intestine, rectum, nose
Sagittarius : Hips, thighs, sciatic nerve, liver, arteries
Capricorn : Skin, knees, hair, nails, teeth, cell walls, joints
Aquarius : Blood circulation, lower legs, calf, ankles, shins
Pisces : Feet, toes, lymphatic system, spleen in its lymphatic role

YIN signs : Taurus, Cancer, Virgo, Scorpio, Capricorn, Pisces
YANG sings : Aries, Gemini, Leo, Libra, Sagittarius, Aquarius

Modality of the signs
It concerns the basic reactions or response patterns of the organism to pathogenic factors and the disease process.

Cardinal :
Illness is usually clear and definite. Remission; often, sudden, dramatic onsets and remissions may alternate. Predisposed to acute disorders, moderatelevel of inherent vitality and resistance to disease.

Fixed :
Highest inherent vitality and resistance to disease.  But once disease gains a foothold, it similarly becomes persistent and tenacious, and often becomes chronic. Eliminative functions are slow and sluggish. Poor detoxification : once toxins, pathogens and morbid tumors gain a foothold, they’re also difficult to expel.

Mutable :
Minor complaints that come and go.  Inherent vitality and resistance to disease is usually lowest. Tend to have a sensitive, allergic nature.The Mutable signs rule the digestive and respiratory tracts, which are always in intimate contact with the outer environment, and therefore allergically and immunologically challenged.

Temperament by element 

Fire signs = Choleric
Earth signs = Melancholic
Air signs = Sanguine
Water signs = Phlegmatic

Ascendant =
Birth, infancy, childhood
MC = Prime of life
Descendant = Maturity and old age
Nadir/ IC = Death and its legacy or aftermath

Planets in 6th house

Your health can easily be seen by your 6th house. Having a planet in that house doesn’t mean you will have a certain illness,
but it increases the risk of it.

Sun in 6th = prone to exertion
Moon in 6th = prone to depression
Mars in 6th = prone to irritation
Mercury in 6th = prone to stress
Venus in 6th = prone to unpleasantness
Jupiter in 6th = prone to exaggeration
Saturn in 6th = prone to burdens
Uranus in 6th = prone to confusion
Neptune in 6th = prone to distraction
Pluto in 6th = prone to destruction

Placement of 6th ruler

The nature of how you catch a disease can be seen by the house in which the ruler of the 6th house is located. So for example, if the Lord of your 6th house is Leo, than the ruler is the Sun, Look where the Sun is placed and if it’s located in the 1st house, it means that you easily catch a disease or illness due to personal (bad) habits.

1st house : personal habits
2nd house : chronic from birth
3rd house : friends
4th house : home and family
5th house : pasttime activities
6th house : work
7th house : partner
8th house : genetic, unknown
9th house : travel
10th house : work, social life
11th house : strangers, community
12th house : weakness, low resistance

The degrees

6 degrees Leo / Aquarius : bad for eyesight
18 degrees : diseases, unlucky degree
25 degrees in Aquarius : linked to alcoholism

29 degrees Taurus : bad for eyesight
9 degrees Gemini : Weak lungs
20 degrees Gemini : Bad for eyesight
9 degrees Libra : Spinal problems
14 degrees Scorpio : Health problems, physical anomalies
25 degrees Sagittarius : Trouble eyesight
23 degrees Pisces : Fever, cuts
28 degrees Pisces : Suicidal tendencies

Various sources such as : http://www.greekmedicine.net