As one is predominant, the other supports. Think of the flame of a candle as yang and the wax as yin. Without the wax, the flame dies. Without the flame, the wax has no purpose. These five phases foster an understanding of how the body functions relate to each other and how qi flows.
Chinese medicine recognizes both internal and external causes of disease.
Diagnosis is the recognition of imbalance in qi. A full delivery system for goods and services is in place and the practitioners are educated with 5 years of college, internship, and duly licensed and regulated. I thought you might be interested in seeing a list of studies that are currently underway in the US investigating the effectiveness of various Chinese modalities. And here is a list of conditions that studies have suggested may be helped by acupuncture. In most cultures, this is a part of the religious practices.