According to Chinese medicine, there are three main ways that the body can become diseased. One way is an internal disharmony of organ functioning, caused by improper diet, overstrain, lack of exercise, or pathological emotions. Another way is an invasion of the body by an external pathogenic factor (epi), and the third way is trauma and injury. There are six External Pathogenic Factors. They are wind, cold, damp, dry, heat, fire, and summer heat. Heat and fire are the same pathogen but differ in degrees of severity. The epis are thought to invade the body through a series of layers, beginning with the exterior (skin) and ending with the interior (internal organs). All of the epis are defined in greater detail individually.