Perspectives in Biology and Medicine, volume 56, number 4 (autumn 2013): 513–529. © 2014 by Johns Hopkins University Press 513 ABSTRACT Diagnosis plays an important role in how we understand disease, and how medicine confirms its status in contemporary society. However, diagnoses are far less concrete than their taxonomies suggest. This essay presents influenza as a case study in the elusive nature of the diagnosis, and in its complicated realities. Using the metaphor of boundary transgression, it reveals the fluidity of diagnosis and the paradox- es presented by the naturalization of diseases. In order to contain influenza, medicine commits other paradoxical transgressions of boundaries. Lay self-diagnosis, use of the lay expression “flu,” and wide reliance upon the belief in the influenza-like syndrome are used to attempt to cement a concrete notion of influenza. D IAGNOSIS IS COMMONLY THE ENTRY POINT to the world of medicine, and it is pivotal to the way Western medicine is practiced, contributing to medi- cine’s authoritative position in contemporary society. It is the pursuit of diagnosis that leads the individual to consult with a medical practitioner. Diagnosis is the mechanism by which treatments are identified and allocated, services accessed, the sick role designated, identity transformed, and medical authority confirmed (Jutel 2011; Jutel and Dew 2014). Diagnosis is not only one of medicine’s most important roles and powerful classification tools, it is simultaneously a means for understanding the practice of medicine. As medicine finds labels to describe illness, it divides the continuity of nature into what Zerubavel (1991) refers to as “islands of meaning.” It is, he Victoria University of Wellington, Graduate School of Nursing, Midwifery and Health, P.O. Box 600, Wellington 6140, New Zealand. E-mail: annemarie.jutel@vuw.ac.nz. When Pigs Could Fly influenza and the elusive nature of diagnosis Annemarie Jutel