In 1888 Émile Roux and Alexandre Yersin demonstrated that the systemic effects of diphtheria — paralysis, heart damage, death — were not caused directly by the bacterium but by a toxin it secreted. It was a finding of immense consequence: for the first time the infectious agent was dissociated from its mechanism of damage. Roux's work opened the way to toxoids — vaccines based on inactivated toxins — and to passive serotherapy, inaugurating applied immunology as a discipline. In the Wikinventia chain, Roux does not compete with Pasteur: he completes and extends him toward a mechanistic understanding of immunity.