What is a disease?
What is it in a human that actually falls sick? 
What do we actually treat with medicines?

Is it the body or the mind?

A disease is not merely a structural or a functional disorder; it could easily be an emotional one also. Beyond the symptom-driven definitions in contemporary medical science, a disease is what disrupts your equilibrium, that is, your physical, mental, emotional and spiritual balance. Self-deprecation is a disease, as is pride; anger is a disease, as is envy. Anything that throws you off-balance, physically or mentally, is a disease. 

The yogic texts refer to the inner man, or the 'self' as purushaPura means city. Ayurveda considers the body is a city of nine gates. The pure consciousness, the vital force that gives us individuality - that gives us the ability to sustain ourselves as a separate entity while being part of nature - is called the inner being or purusha. This is our centre of vitality, the control centre. Anything that afflicts the inner man is a disease; that disease has its primary seat in the inner spring of vitality from which it flows out to the surface, the external body. 

According to Sushruta, father of Ayurvedic surgery and author of Sushruta Samita:

‘In man, as in everything else in the universe, the direction of the inherent force is from the centre to the circumference. The shock is felt first at the center of vitality, whence it is transmitted outwards and thus affects the energy which holds the molecules together, dvyanuka and tryanuka (binary and tertiary atoms) of which the physical body is composed, and further opposes the dissolution of those molecules into their elemental constituents in the living organism. 

Even in cases of external injuries such as snakebite, etc., the potency of the virus is carried at once to that center from whence it is almost instantaneously transmitted through the external channels of the body to its surface, otherwise what purpose does vayu (vital energy or nerve force) serve in the human economy? What do those myriads of chaitanya-vahini nadis (sensory nerves, literally, nerves carrying consciousness) exists for in the human system? 

In all diseases the subjective sensations are the first to be experienced. ‘I am ill’, ‘I feel hot’, etc., are the voices of sensations, which form the basis of the disease. 

Disease then is a force and not matter.’

Anything that afflicts the inner being is a disease. The inner being is the one that experiences pain and pleasure, joy and sorrow; the one that is the essence of your life. The state of your physical health and the state of your mental health both affect the inner being. 

Disease is a force and not matter. Both diseases and afflictions arise out of resistance. Disease is a resistance of the opposing forces in your body. The result of this resistance could be anything from formation of mucus or pus, to more serious cysts or ulcers, to terminal illnesses. Although you are born with a certain constitution - and this is a powerful factor in determining your degree of resistance - the truth is that your health is chiefly in your own hands. 

You can elevate your physical and mental states so that you become resistance free. When there is no resistance with the forces of nature, when there is no inner conflict of emotions and thoughts, you become healthy naturally.