Yin & Yang are the 2 aspects of vital energy. Energy has 2 polarities. They are the basic principle of entire universe and function as two opposite forces like positive polar and negative polar. They are complementary to each other and balance each other but at the same time they are opposite to each other. Thousands of years before Newton proved that action and reaction are equal and opposite, the Chinese had the concept of Yin & Yang.
Without day there cannot be night and without night there cannot be a day. Interdepending relation of the two is necessary for the existence of either. Nei Jing says that the universe is a state of oscillation of forces of Yin &Yangand their changes. Because Yin & Yang relate to each other in harmony, balance exists in the body. Good health is a state of energy balance between Yin & Yang. Upset in the balance of Yin & Yang is a disease.
As long as there is a balance between Yin &Yang, life is smooth. Natural disasters like earthquakes, floods, volcanic eruptions, famine are brought about by an imbalance of the Yin &Yang forces in the universe.