In the purest and transformative senses of the word, revolutions do not happen often. For an event to be revolutionary, by definition, it cannot happen every day. Thus, when Porter argues that bacteriology created a revolution, it is an exception because such a transformative event could not happen every day. It is an exception to the rule because such revolutionary change is rare. Porter makes this case evident in his analysis.
Initially, it needs to be argued that Porter feels that the studies in bacteriology were groundbreaking. He felt that they took place in a particular moment in time, and reflected a condition that was unique for its historical and scientific instant:
Nineteenth-century medical science made spectacular leaps forward in the understanding of infectious diseases. For many centuries, rival epidemiological theories had attributed fevers to miasmas (poisons in the air, exuded from rotting animal and vegetable material, the soil, and standing water) or to contagion (person-to-person contact). From the 1860s, the rise of bacteriology, associated especially with Louis Pasteur in France and Robert Koch in Germany, established the role of micro-organic pathogens. Almost for the first time in medicine, bacteriology led directly to dramatic new cures.
Bacteriology was profound for Porter because it represented the convergence of the Enlightenment. Its emphasis on the scientific method was embedded in society, representative of how the pivot to bacteriology took place a century after the Enlightenment. Porter feels that the emergence of bacteriology as a means to develop "dramatic new cures" was groundbreaking and would become the standard in medical development. Porter feels that bacteriology was groundbreaking because it established a base from which all medical advances would originate. From bacteriology, medical scientists recognized the nuanced nature of infectious diseases, causes for contagion, and the role of micro- organisms in health science advancement.
Porter sees this development as revolutionary. He also sees it as the exception to the rule in so far as such a transformation could not happen again. Porter sees the understanding of bacteriology as forming the basis of all medical advances today. The study of bacteriology would form the basis of medical advancement because it combined the knowledge base of the Scientific Revolution as well as the anatomical focus of the Renaissance. For Porter, such a transformative nexus could not happen again because it became the starting point from which all future medical endeavors would originate. Bacteriology becomes the exception to the rule because it ends up being appropriated by the medical community as where all medical pursuit should begin. Since its introduction, medical science has recognized the function of bacteria study as well as the purpose and meaning of microorganisms. Revolutionary as it was at the time, it has now become the staple of modern medical research. Its findings start from here, and thus it is no longer revolutionary and groundbreaking. It has become standard. For this reason, it has become the exception to the rule when it comes to scientific research and medical advancement. When Porter refers to the transformation as one of medicine's "few revolutions," it is meant in this regard.