1884

Pasteur published technique of increasing virulence of ordinary canine rabies by serial passages through guinea pigs or rabbits, and then discovery of an organism in which serial passages produced the opposite effect, decreasing the virulence of rabies: “Successive passages of the saliva microbe through the guinea pig, for example, made the microbes less virulent for rabbits . . . the virus could be attenuated for dogs by passing it from dog to monkey and then successively from monkey to monkey. . . At some point in its serial passage through monkeys, the rabies virus lost its virulence for dogs and began instead to protect them from the effects of somewhat more virulent strains of the virus, which in their turn acted as vaccines against still more virulent strains until eventually dogs could be rendered immune to even the most lethal virus” (Geison, 191).