In 1822 Gaspard showed the poisonous nature of material from infected wounds by injecting it into animals and causing their death. Tiedemann (1822), Peacock (1828) described “little bodies” in the muscles of human cadavers which Hilton (1832) considered to be parasitic in nature. Paget (1835) showed that these bodies were round worms and Owen (1835) described them more accurately and gave the name _Trichina spiralis_ to them. Leidy (1846) found organisms in the muscles of hogs which he considered to be the same as Owen’s Trichina and paved the way for the work of Zenker (1860) in showing the pathological relation between the Trichina of pork and human Trichinosis. Bearing on the “contagium vivum” theory was the rediscovery of the “itch mite” (_Sarcoptes scabiei_) by Renucci (1834), an Italian medical student. This had been declared several hundred years before but had been lost sight of. Chevreuil and Pasteur, in 1836, showed that putrefaction did not occur in meat protected from contamination, and suggested that wound infection probably resulted from entrance of germs from without. Bassi, investigating a disease of silkworms in Italy, demonstrated that a certain mold-like fungus (_Botrytis bassiana_) was the cause in 1837. This was the _first instance of a microscopic vegetable organism_ proved to be capable of _causing disease in an animal_. Boehm, in 1838, observed minute organisms in the stools of cholera patients and conjectured that they might have a causal connection with the disease. Dubini of Milan in 1838 discovered the _Ankylostoma duodenale_ which later was further described by Omodei in 1843 and shown to be the cause of Egyptian chlorosis by Griesinger (1851). The fungous nature of favus, a scalp disease, was recognized by Schönlein in 1839, and the organism was afterward called “_Achorion schoenleinii_.” Berg, in 1839-41, showed that thrush is likewise due to a fungus, “_Oidium albicans_.” These discoveries led Henle, in 1840, to publish a work in which he maintained that all contagious diseases must be due to living organisms, and to propound certain postulates (afterward restated by Koch and now known as “Koch’s postulates” p. 233) which must be demonstrated before one can be sure that a given organism is the specific cause of a given disease. The methods then in vogue and the instruments of that period did not enable Henle to prove his claims, but he must be given the credit for establishing the “contagium vivum” theory on a good basis and pointing the way for men better equipped to prove its soundness in after years. [Illustration: PLATE II SIR JOSEPH LISTER]