Pathogenic factors can invade the body in various forms and are classified by Chinese medicine as:
In TCM literature, they are sometimes referred to as the Six Pernicious Influences (Liu-Yin) or Six Evils. They are considered to be meteorological or climatic factors which play a part in disease. Each of the above pathogenic factors can be of exterior or interior origin, however, they are generally considered to relate mainly to outside influences. They generally correspond to a full pattern according to the Eight Principles.
The invasion of the body by a pathogenic factor usually arises as a result of an imbalance of Yin-Yang.