Do you know what the symbol for Yin and Yang really means? It is not about good vs. evil, nor is it about a bit of bad in the good and a bit of good in the bad. It is more complicated than that and it has a distinct spiritual meaning that has found its way into Western culture, especially in alternative/complementary/integrative modalities.
Due to the fact we are in a post-truth age where the idea of absolutes and checking out facts is vanishing, too many things like functional medicine are flourishing. This is one reason there is a profliteration of New Age ideas and practices.