The Organs (nourished by Nutritive (Ying) Qi.
Interior illness is produced by disharmonies of the Internal Organs, which may be due to either Internal or External causes. For example after you get a cold you may develop a cough. The cough occurs because the illness has affected the lungs and has now become more internal. Interior illness is generally caused by deficiency or by stagnation.
More specific principle
Deficiency or Stagnation caused
Questions to ask
The focus of this diagnosis is the relative depth of the disease in the body. When speaking of the body in TCM, the Exterior refers to the skin, muscles and Channels. The Exterior Qi is the relatively Yang Defensive (Wei) Qi. The Interior refers to the Organs, which are nourished by the relatively Yin Nutritive (Ying) Qi.