into the lab animals, they were able to observe all the typical signs of the sickness. Roux and Yersin established that they were dealing with a new type of bacillus, not only able to proliferate and abundantly reproduce itself, but also capable of spreading at the same time a powerful venom, and they deduced that it can play the role of antigen, that is if they could overcome the delicate moment of its injection, made especially dangerous by the toxin. Some German researchers had also discovered the diphtheria toxin and were trying to immunize some guinea pigs through the use of a vaccine: one of them, Von Behring, Robert Koch's student, stated that he was able to weaken small doses of the toxin. Nonetheless, Roux was not convinced by this result, since no one knew the collateral effects of the procedure, and preferred to use serotherapy since more than one lab study – like the one accomplished by Charles Richet – demonstrated that the serum of an animal vaccinated against the disease included the antibodies needed to defeat it. The anti-diphtheria serum which was able to agglutinate the bacteria and neutralize the toxin was supplied by a horse inoculated with the viral germs, and it was separated from the blood drawn from the horses’ jugular vein. Roux needed to test the effectiveness of the product he elaborated on To test the serum, two groups of children were chosen from two different hospitals: in the first one, which received the serum, 338 out of 449 children survived, in the latter one, treated with the customary therapies, only 204 out of 520 survived. Once the results were made public by Le Figaro newspaper, a subscription fund was opened to raise the money needed to provide the Institute with the number of horses necessary to produce enough serum to satisfy the national demand. After Duclaux's death, Roux took his place as head of the institute, and the last research he carried out was the one on syphilis, a dangerous disease because of its immediate effects and the hereditary repercussions that result from it. Despite Fournier’s considerable work, van Swieten’s liquid mercury was still the only known cure, although its results were doubtful and uncertain. The search for a stronger remedy against this disease was made more difficult because most animals are immune to it: it was thus not possible to experiment with possible