This article analyses the proceedings of eight International Sanitary Conferences which were convened between 1851 and 1894 to address the danger that cholera epidemics posed to Europe. These conferences are examined in the context of the intellectual and institutional changes in scientific medicine and in the light of the changing structure of internationalist endeavours that took place in the second half of the nineteenth century. The article shows that the International Sanitary Conferences were as much spaces of co-operation as they were arenas where differences and boundaries between disciplines, nations, and cultures were defined. Furthermore, it seeks to shed light on a broader tension of the period. On the one hand, the fact that the world was growing together to an unprecedented extent due to new means of transportation enabled Europeans to establish and expand profitable commercial and colonial relations. On the other hand, this development increased the vulnerability of Europe – for example to the importation of diseases. The perception that the world was becoming increasingly interconnected was thus coupled with the need for controllable boundaries. The conferences attempted to find solutions as to how borders could be secured without resorting to traditional barriers; like semipermeable membranes they should be open for some kinds of communication but closed for others.