There are several different factors that affect health and happiness including nutrition, lifestyle, exercise, stress, family and career. However the one big factor that in my experience often gets overlooked is the health of the mind. It seemed to me that in order to create and maintain a healthy and happy life the work must begin with the mind.
Your thoughts are extremely powerful and can control all aspects of your life including your health. The majority of people are only partially aware of the thoughts that go through their head.