A theme throughout The Invisible Kingdom, as is indicated by the title, is that of the feeling of invisibility wrought by unnamed chronic illness. By naming the invisibility, O’Rourke strives to make her struggle visible to others, and thus her book stands as testimony of a life characterized by struggles that others would prefer not to acknowledge. The theme of invisibility relates to both the social and private experience of O’Rourke’s illness. For O’Rourke, her illness transforms her life and sense of self. However, without a viable diagnosis, this transformation remains invisible to those around her. She writes, “At times I thought I would slowly slip away unseen and no one would notice because the shell of my body was still there” (100). The dissonance between the profound presence of her symptoms and the invisibility of their manifestation creates a sense of loneliness for O’Rourke throughout the text. As she embarks upon this quest for answers and clarity while handling the full-time job of managing her illness, she writes that “it is painfully clear that the invisibility of my illness was one of the most challenging parts of my suffering” (392). While a reader might assume the main conflict of O’Rourke’s experience to be the pain wrought by her illness, it is actually the lack of recognition that poses the most serious threat to her sense of well-being and hope.