The late nineteenth and early twentieth centuries saw a proliferation of laws in colonial India which targeted women deemed to be prostitutes. As the number of laws grew, so too did the category of ‘prostitute’. Yet, before the nineteenth century, it would have been difficult to identify many of these women or their activities as criminal, or even immoral. This article examines how such legal boundaries and conceptualisations came to be formulated. It suggests that the ‘prostitute’ category in India was shaped by the repeated failure of the East India Company's surgeons and officers to control venereal disease among the European soldiery. Such attempts at disease control were experimented with from the late eighteenth century and, as this article argues, were keys in the later formulation of the Contagious Diseases Acts. This article traces the decline of long-term, monogamous relationships between European men and Indian women, and the corresponding rise in shorter-term sexual transactions in and around military cantonments. By grounding later legal shifts within the military medical context, we can clearly see the forces behind the social and moral changes surrounding this group of women in the early nineteenth century.