Let us talk about the most common illnesses, like tumors, infections or osteoproliferation etc., found in the human body. In another space, a very deep space, there lies an evil intelligent entity. An ordinary qigong master or a common supernatural capability is unable to see it except for the black qi in one's body. It is correct to say wherever there is black qi there is an illness. Yet, the black qi is not the fundamental cause of a disease but rather the field that is released by an evil intelligent entity from a deeper space.
Some people talk about discharging and expelling the black qi.
It will be produced shortly. Some are very powerful. It will be pulled back right after being expelled. They can draw back qi themselves, which makes treatment impossible. From the perspective of supernatural capability, the black qi in that area is considered to be the qi of an illness.