Having elucidated the essence of inflammation, he wished to study the origin of another pathological symptom, _i.e._ the rise in temperature which constitutes a feverish condition. To that end he undertook a succession of experiments on cold-blooded animals; he injected microbes into crocodiles and serpents, hoping thus to provoke a rise in their temperature. But those experiments did not give the results expected. In the meanwhile (1892) cholera had made its appearance in France; the specificity of the cholera vibrio was not finally established at that time. The observations made by Pettenkoffer on the immunity of certain regions, despite the presence of the cholera vibrio in the water, and the experiments made upon himself by that scientist, seemed to plead against the specificity of the cholera vibrio; but other facts spoke in its favour. Desirous of solving this question, Metchnikoff went to a cholera centre in Brittany in order to fetch the necessary materials. Having done so, he attempted to produce cholera in divers kinds of animals, but without success. As he failed to solve the problem of the specificity of the cholera vibrio on animals, he resolved to experiment upon himself and consumed a culture of cholera vibriones. He did not contract cholera, which made him doubt the specificity of the vibrio, and therefore he consented to repeat the experiment on one of his workers (M. Latapie) who offered to submit to it: the result was the same. He then did not hesitate to accept the offer of a second volunteer (M. Jupille). The preceding results having led him to suppose that the cholera vibrio became attenuated _in vitro_ and might perhaps serve as a vaccine against cholera, he gave a culture of long standing to the young volunteer. To his astonishment and despair, Jupille began to manifest the typical symptoms of cholera, and a doctor who was particularly conversant with the clinical chart of the disease declared the case a severe one because of the nervous symptoms which accompanied it. Metchnikoff was in mortal anxiety, and even said to himself that he could not survive a fatal issue. Fortunately the patient recovered, and this terrifying experiment proved indisputably the specificity of the cholera vibrio. Yet the irregularity of its action showed that in certain cases conditions existed which prevented the inception of the disease, and Metchnikoff supposed that this might be due to the action of the different intestinal micro-organisms. In order to simplify the question, he began by making experiments outside the organism. He sowed the cholera vibrio with divers other microbes and saw that some of them facilitated its culture whilst others prevented it. Similar experiments within the organism of animals gave no conclusive results; the simultaneous ingestion of the cholera vibrio and of favourable microbes did not induce cholera.