Serotherapy represented a major advance in the fight against infectious diseases. Medical doctors and veterinarians permanently collaborated both in the invention of this new therapeutic procedure in the 1890’ s and in its use, especially during the World War I. The emergency of serotherapy resulted from the work conducted in parallel on diphtheria by Emile Roux’s group at the Institut Pasteur and Emil Behring’s group in the laboratory of Robert Koch in Berlin. It consisted in immunizing horses with partially inactivated diphtheria toxin and then using the serum from these horses, which contained antitoxin (it was the discovery of the first antibodies) to treat children with diphtheria. Applied to the prevention of tetanus to patients suffering from deep wounds, serotherapy saved the lives of millions of soldiers during World War I. It was also applied to cure or prevent other infectious diseases such as plague, dysentery or gangrene, and in the protection against the toxins from snake venom. It constituted a first step in the development of some of the most widely used vaccines, such as those against diphtheria and tetanus. Since it was not exempt from serious side effects, serotherapy was later supplanted by antibiotics. However, it is now making a spectacular comeback with the use of monoclonal antibodies to treat various types of diseases.