Laboratory manipulation of the virulence of pathogenic microbes had been a central component of the birth and life of Pastorian microbiology. Pasteur and his disciples had fashioned their assorted vaccines by exposing microbes to a variety of changed environmental conditions—heat, cold, air, and so on.They had also found that passage through animal hosts tended, eventually, to restore such artificially diminished virulence. Given this practical focus on microbial malleability (along with other cultural and certainly personal factors), it is unsurprising that Pasteur did not himself come up with a formula for a strict, “one microbe produces one disease” specificity. Pasteur and Koch roughly agreed on microbial specificity; Pasteur was simply willing to admit more flexibility within a species. Mazumdar has argued that it was Pasteur's disciple and successor at the institute's helm, Emile Duclaux, who attributed the “discovery” of disease specificity to Pasteur. In fact, as Geison notes, although Pasteur's interest in the manipulation of microbes did not go so far as challenging the borders of their “species,” he was, in theory, willing to push the borders of disease species still further, even suggesting that the relationships between hosts and parasites evolved over time—and that there would be new diseases. Emile Roux also raised the possibility of disease evolution in his Cours; however, neither he nor Pasteur pressed it much further. Charles Nicolle did.

In 1930, Nicolle published his classic treatise on disease evolution, Naissance,Vie et Mort des Maladies Infectieuses [NVM]. He wrote much of the book while recuperating from his long illness in 1929. In the text, the invisible forces of inapparent infection and inframicrobes found central positions— as did his father's natural-historical inclinations and his brother Maurice's immunological models.