The mind is very subtle, it permeates every atom of your body and extends through many layers of your existence. Your mind is the creator of your happiness and suffering. Your understanding of reality and experience is through your perception, through your mental state.
Core nature of the mind is pure light, true knowledge, peace and serenity. But because of the external influences, conditions and habitual tendencies, the mind follows certain patterns according to which it tends to function. If these influences are negative, your thinking pattern will take on the negative form. The habit of negative thinking draws out the energy of your body and leads you closer to disease.
Therefore, you need to cultivate your mind towards positive patterns and influences by choosing a positive environment, repeating affirmations and doing meditation.