In mediaeval times, pestilences were spread by personal infection prevailed in practice, notwithstanding the teaching of venerable and orthodox authority. Sydenham, for instance, admitted infection in Plague, but denied it for Small-pox. Although Mead and some physicians as well as those in control of public action accepted the doctrine of contagion, in part at least, it was denied by anti-contagionists so recently as the seventh decade of the nineteenth century, when Pasteur's discoveries made this position almost untenable. The stages in the progress towards an appreciation of contagium vivum have great historic interest. This chapter also outlines the groping after truth, and the successes and failures in preventive medicine which characterised the first six or seven decades of the nineteenth century. It finally traces the course of events up to the period initiated by Pasteur's immortal work.