Among the most prominent opponents of vaccination were two members of the medical profession, one more eminent than the other. Edgar Crookshank, Professor of Comparative Pathology and Bacteriology and Fellow of King's College, London, wrote a History and Pathology of Vaccination in two volumes (1889). The Preface to the first volume described how, following some investigations into an outbreak of cowpox, he became convinced that

The essence of Crookshank's argument against vaccination was that the power of conferring immunity to smallpox, claimed for cowpox by disciples of Jenner, was a fallacy because what was injected under the guise of cowpox was in fact smallpox. The origins of this controversy lay in the earliest period when, with supplies of cowpox very scarce for lack of suitable outbreaks to draw on, some confusion had occurred over a consignment of alleged cowpox despatched by Jenner from London to a colleague in the country.