"It was upon the above line of reasoning (in these reports), that I conceived the idea that the yellow-fever germ must be conveyed from the patient to the non-immunes by inoculation, a process which could be performed in nature only through the agency of some stinging insect whose biological conditions must be identical with those which were known to favour the transmissibility of the disease." In 1881 he inoculated himself and six soldiers with infected mosquitoes, and obtained, as he had calculated, mild attacks and subsequent immunity. During the years 1881-1900 he inoculated by this method 104 persons:-- "In these inoculations, be it remembered, my principal object was rather to avoid than to seek the development of a severe attack; in point of fact, only seventeen showed any appreciable pathogenic effects after their inoculation. I felt sure, however, that severe or fatal result might follow an inoculation either with several mosquitoes contaminated from severe cases of the disease, or from a single insect applied several days or weeks after its contamination, having come to this last conclusion in view of the facts connected with the _Anne Marie_, and the epidemic of Saint Nazaire." Dr. Finlay's discovery that the mosquito can convey yellow fever, and that the germ of the disease is more virulent after a prolonged sojourning in the mosquito, was proved beyond all question by the work of 1889-1901. But, so far as immunisation is concerned, few people would submit themselves to be bitten by an infected mosquito, even with perfect assurance that the germs contained in it were of a low degree of virulence: the urgent need, therefore, was for an immunising serum. In 1896, at Flores, Sanarelli discovered the _bacillus icteroides_; and by October 1897, he had prepared an immunising serum which was able to give a considerable amount of protection to animals.[40] Next year (_Annales de l'Institut Pasteur_, May 1898) came the news that he had advanced against yellow fever with its own weapons--_Premières expériences sur l'emploi du sérum curatif et préventif de la fièvre jaune_. Of the first 8 cases (Rio de Janeiro), 4 recovered. Then came the 22 cases at San Carlos do Pinhal, in Saint-Paul au Brésil (January 1898), with 16 recoveries, and only 6 deaths. And it is to be noted that he submitted his method of treatment to the utmost test that was possible; he chose the bad cases, and the country where the fever was most fatal:-- "Chaque cas était choisi de commun accord entre nous, dans le but de mettre bien en évidence l'action thérapeutique du sérum, _mettant toujours de côté tous les cas qui se présentaient avec des symptômes vagues ou attenuès ou en forme légère ou fruste. On ne conservait donc que des cas oû, d'après la violence des phénomènes d'invasion, on devait considérer comme très peu probable une crise spontanée de la maladie_...."