cures and study their likely side effects. The sexually transmittable Treponema pallidum (the syphilis germ), detected by two German biologists, Schaudinn and Hoffmann, affects only the human racewhere it resides in sperm, ulceration, and cancers that it is able to causeand, as it would later be discovered, some anthropoid apes, especially chimpanzees. Both Roux and Metchnikoff, following the discovery that this type of ape can be contaminated with the illness, contributed with their research in creating a vaccine, while Bordet and Wassermann elaborated a solution that was able to expose the germ's presence in human blood. Even though it was not yet a completely reliable solution, it represented progress compared to the previous medicines used against syphilis. Metchnikoff's phagocytosis theory Ilya Ilyich Mechnikov had already published the "principle of immunization" during his voluntary exile in Italy, where he went to undertake some studies, the results of which he had promptly communicated to Pasteur. The phagocytosis theory is based on the notion that phagocytes are cells that have the power to englobe foreign bodies – and above all bacteria – introduced inside an organism. German biologists opposed to his doctrine the humoral theory: they claimed to have found in Roux's serum some substances able to reveal the presence of microbes, and to ensure their destruction if properly stimulated. The German scientist Eduard Buchner referred to these substances as "alexine" and two other biologists, Von Behring and Kitasato demonstrated their lytic power towards bacteria. In 1894 one of these scientists published the result of an experiment that appeared to refute Metchnikoff's ideas: using the cholera vibrio, discovered ten years before by Robert Koch, as an antigen, Richard F. J. Pfeiffer introduced it in the abdomen of a guinea pig already vaccinated against this disease, and was able to observe the destruction of the vibrio in the local blood plasma, without the participation of the phagocytes. Not even this study was able to shake Metchnikoff's belief and faith in his theory, and his ideas, as well as Pfeiffer's and Buchner's, would all contribute to the elaboration of the current theory of the immune system. Yersin's studies on the plague Yersin, after his research with Roux, abruptly left the Institute for personal reasons. The news of a violent plague outburst in Yunman enabled Yersin to show his potential as he was summoned, as Pasteur's scholar, to conduct microbiological research of the disease. The plague