Whilst inoculating some rabbits and dogs with a microbe taken from pus (_Staphylococcus pyosepticus_), I developed a certain disease both in the rabbits and in the dogs. But the dogs did not die, whilst all the rabbits died from the results of the inoculation. I thought then that, the cause of that resistance being due to the difference of blood, I might be able to make the rabbit refractory to the infection by injecting it with the blood of a dog in normal health. The experiment succeeded. The rabbits which had received the blood of the dog, when they were afterwards infected with the staphylococcus, became very ill but did not die. Later on, I took, not the blood of a dog in normal health, but the blood of a dog that had received the infection of the staphylococcus and had recovered from that infection, and I injected this blood into the rabbits. _Now the rabbits that received the blood of the infected, healed dog had acquired complete immunity to this form of microbe infection_: the principle of serotherapy was discovered (5th Nov. 1888). [Illustration: "L'ENFANT." _In Musée du Luxembourg, Paris._ _facing p. 53._] Since then, serotherapy has been applied, by Behring in Germany and by Roux in France, to diphtheria (1892). These two savants showed that the blood of animals, and especially of horses, that had been infected with diphtheria and cured, could, when injected into patients attacked by diphtheria, diminish, in an extraordinary proportion, the duration and intensity of the disease. There is no other treatment for diphtheria to-day. A doctor is guilty, and even criminal, if he does not use it, for the therapeutic results of this treatment are marvellous. I do not speak of clinical observation only. All those who have seen the effects of one of these injections of serum on children down with diphtheria are veritably stupefied at the resurrection which they witness only a few minutes after the injection. The unfortunate child with his purple face and convulsed limbs, scarcely breathing, comes back to fresh life as soon as he has received the beneficent injection of serum. The facts are so decisively clear that even if we have only seen them once we can never again forget them. But I shall simply call the attention of my readers to the following statistics, the result of more than 500,000 observations made in England, in the United States, in France, in Russia, in Germany, in Italy, in Austria, in fact everywhere: the death-rate in diphtheria before 1892 (for the serotherapic method took four years to become known and practised) was 45 per cent. After 1892, this death-rate fell to 12 per cent.[8] Consequently, out of every hundred patients suffering from diphtheria, thirty are saved by the serotherapic treatment.