On return from Egypt, Crookshank toured Europe in 1884 for further medical training. In Berlin, he visited the laboratory of Robert Koch and learned methods of isolating bacterial strains to investigate infectious diseases.[citation needed]
When he returned to London, Crookshank wrote a textbook, An Introduction to Practical Bacteriology Based on the Methods of Koch, which was published in 1886. Subsequent editions were published under differing titles in 1887, 1890 and 1896, and a French translation by H. Bergeaud was published in Paris as soon as 1886.[citation needed]
In 1885, Crookshank founded one of the world's first bacteriological laboratories for human and veterinarypathology in London.[1][2]
Crookshank was also interested in the use of photography to study bacteria and published Photography of Bacteria in 1887, the first text in English devoted solely to the photography of bacteria.[2] In the introduction to this book he wrote that the photographs were "intended to convince scoffers of the essential truth of the new Science, that specific, often morphologically distinct, microorganisms were the cause of particular infectious diseases".
During this time he became interested in the study of infectious diseases in animals and in 1886 was awarded the chair of Comparative Pathology and Bacteriology at King's College London. In his new role he was asked to investigate an outbreak of cowpox in Lechlade, Gloucestershire.
His investigations led him to reconsider the use of cowpox-derived vaccines to immunize against smallpox, a treatment developed by Edward Jenner nearly a hundred years earlier. His conclusion was that such vaccines were ineffective in preventing smallpox because the two diseases (cowpox and smallpox) were "totally distinct".[citation needed] Instead of a cowpox-derived vaccine, he advocated the use of a more dangerous vaccination using attenuated smallpox. In 1889, he published a two-volume treatise on the subject, A History and Pathology of Vaccination. Vaccination policies were a divisive topic at the time and in the ensuing controversy that resulted from his publication, Crookshank quit his chair at King's College London in 1891. He continued to speak out on health matters, but never worked in a laboratory again. He subsequently, however, focused on the encouragement of agricultural and veterinary science, serving as a governor of the Royal Veterinary College until his death.[citation needed]
A Textbook of Bacteriology: Including the Etiology and Prevention of Infective Diseases and a Short Account of Yeasts and Moulds, Haematoza, and Psorosperms, Nabu Press, 2010, ISBN978-1-174-75161-5
