Commentary: confronting unexpected results: Edmund Parkes reviews John Snow.

1 is one of the most famous works in the history of epidemiology. It first appeared as a modest pamphlet in 1849 in the midst of Britain's second epidemic of cholera. A second and more substantial edition appeared in 1855 following another but smaller outbreak , and it is this second edition for which Snow is remembered today. Edmund Alexander Parkes was a logical choice to review Snow's second edition. As a member of the Army Medical Service in India and Burma, Parkes had seen cholera where it was en-demic, and he had published substantial pieces on the pathology and therapy of cholera, dysentery and hepatitis. When he returned to England in the middle 1840s and received his appointment as an assistant physician at University College Hospital, London, he had a reputation as an up-and-coming medical authority on epidemic diseases and hygiene. By 1855, when he reviewed Snow, he was first physician at University College Hospital and Professor of Clinical Medicine at University College. He later became Professor of Military Hygiene at the Army Medical School at Netley. Before his early death at age 57, he would serve as editor of the British and Foreign Medico-Chirurgical Review, as Goulstonian Lecturer to the Royal College of Physicians and as a member of the General Medical Council, and be elected Fellow of the Royal Society of London. 2 In short, Parkes was an established authority on epidemic diseases and hygiene. In fact Snow cites Parkes's pathological work in his second edition. 3 Parkes's views were certainly informed and mainstream in the mid 1850s. His critique of Snow's work is consequently useful in helping us understand how Snow's contemporaries reacted initially to his cholera theory, and perhaps more importantly, it underlines the magnitude of the conceptual changes Snow's work represents. In order to fully understand Parkes's review, it is important to realize that when Asiatic cholera appeared in Britain for the second time in 1848–49, the newly-created General Board of Health engaged Parkes to undertake a study of the first cases to appear in London. The idea was to use these cases