When we are going to plant something in the garden we take a seed and place it in the soil. According to how healthy the seed is and how receptive the soil is, we will either get a plant that flourishes or a little plant that bears no fruit or does not even start to grow. Human illness is also like that. If we get pneumonia, TB, or meningitis, a seed from outside has come into us and we call it a germ, bacteria, parasite or virus. When it comes in it will either grow or it will not grow. We will become sick or remain healthy. We usually cannot avoid getting the seed in us, but we can avoid becoming sick through the factor of constitution or resistance to illness.
Whether we discuss psychological, emotional or physical illness, the human being can have this inbuilt resistance. The soil is not receptive to the seed so that it doesn't grow. In an office nine people may be working and a tenth comes in coughing germs or viruses over everyone. Three do not get sick at all even though they have breathed in germs. Three get a little sick, running nose, etc. Three spend a week in bed. The secret of this is constitution. Constitution is the basis which makes the soil in us either sufficiently strong so that we avoid contracting any illness or else so weak that we are overcome by bacteria and also by emotional and mental stresses in our environment. What weakens the constitution? Many things, for example, physical and mental problems lower our vitality and if we are low in vitality then our resistance is low. If we are under mental stress, if we have tight spots in the spine, and muscles that are in spasm, if we have pressure on all sorts of nerve roots, then our resistance drops.
Yoga increases physical and mental fitness and vitality. It is only natural therefore that yoga can be used to prevent and treat disease. Yoga deals with building up the constitution of the soil and rendering us immune to these diseases.
It is interesting that modern medicine has concentrated to a great extent on the seed aspect. Usually westerners tend to deal with the external environment more, changing it to suit themselves. Whereas easterners tend to adapt to the stresses of the environment. Now the world is entering an era in which these two are coming together. Medical science, on the treatment of illness, is one of the areas in which this is happening. There is probably no substitute for the work that has been done in eradicating smallpox, malaria, etc. but there is certainly a substitute for our way of life and what is happening within this human soil.
This is constitution, the area where yoga is making vast inroads into the prevention and cure of illness. We are seeing in research laboratories that medical scientists are producing clear, totally acceptable, verifiable, reproducible proof that yoga can cure illness. There are many other areas in which yoga will change our lives, but the one in which yoga is emerging as a significant force is personal health.