The discoveries of Leeuwenhoek which proved the existence of microscopic organisms soon revived the “contagium vivum” idea of Kircher. Nicolas Andry in a work published in 1701 upheld this view. Lancisi in 1718 advanced the idea that “animalcules” were responsible for malaria, a view not proved until Laveran discovered the malarial parasite in 1880.[1] Physicians ascribed the plague which visited Southern France in 1721 to the same cause, and many even went so far as to attribute all disease to animalcules, which brought the theory into ridicule. Nevertheless the “contagium vivum” theory survived, and even Linnaeus in his _Systema Naturæ_ (1753-6) recognized it by placing the organisms of Leeuwenhoek, the contagia of diseases and the causes of putrefaction and fermentation in one class called “Chaos.” Plenciz, a prominent physician and professor in the Vienna Medical School, published in 1762 a work in which he gave strong arguments for the “living cause” theory for transmissable diseases. He taught that the agent is evidently transmitted through the air and that there is a certain period of incubation pointing to a multiplication within the body. He also believed that there was a specific agent for each disease. His writings attracted little attention at the time and the “contagium vivum” theory seems to have been almost lost sight of for more than fifty years. Indeed, Oznam, in 1820, said it was no use to waste time in refuting hypotheses as to the animal nature of contagium. Isolated observers, were, however, keeping the idea alive, each in his own locality. In 1787 Wollstein, of Vienna, showed that the pus from horses with glanders could infect other horses if inoculated into the skin. Abilgaard, of Copenhagen, made similar experiments at about the same time. In 1797 Eric Viborg, a pupil of Abilgaard’s, published experiments in which he showed the infectious nature not only of the pus but also of the nasal discharges, saliva, urine, etc., of glandered horses. Jenner in 1795-98 introduced vaccination as a method of preventing smallpox. This epoch-making discovery attracted world wide attention and led to the overcoming of this scourge which had devastated Europe for centuries, but contributed little or nothing to the question of the causation of disease. Prevost’s discovery of the cause of grain rust (_Puccinia graminis_) in 1807 was the _first instance of an infectious disease of plants_ shown to be _due to a microscopic plant organism_, though not a bacterium in this case. Doubtless one reason why the work on glanders and grain rust attracted little attention among the practitioners of human medicine was owing to the prevalent belief in man’s complete separation from all lower forms of life. The evolutionists had not yet paved the way for experimental medicine.