contagium

plural contagia

1. archaiccontagion; contagious matterFirst known use ca. 1870—1910

   And its escape [from certain diseases] is an approximative proof that, at least for those ten years, no contagium of measles, “nor any contagium of scarlet fever, nor any contagium of small-pox, had arisen spontaneously" within its limits.

   1865, John Simon, “Minutes of Evidence Taken Before the Cattle Plague Commissioners”, in Report on the Origin, Propagation, Nature, and Treatment of the Cattle Plague, published 1866, page 42

   1 more example

   That this in fact was the case was ascertained by Dr. Edington, who, by adding a large percentage (33 per cent.) of glycerine to the gall taken from a rinderpest beast, was able to show that by such contact the infective power of bile was as effectually destroyed as was the contagium of blood.

   1901 December 20, H. Watkins-Pitchford, “Rinderpest”, in The Agricultural Journal and Mining Record, volume 4, number 21, pages 641–642