Our current understanding of host-pathogen coevolution has a strong theoretical base (e.g. Anderson and May 1991) and growing empirical support (e.g. Fenner 1983). In brief, hosts evolve in ways that minimize the fitness costs of infection while, in general, pathogens evolve in ways that maximize their infectivity or transmissibility, which is often expressed as the R0, or basic reproductive ratio, of the pathogen. The virulence of pathogens is an outcome of this coevolutionary process. Most of us who focus on the recent literature may not appreciate that these ideas were first presented by Hubert Airy over a century ago, in a remarkable paper “On infection considered from a Darwinian point of view” (Airy 1882). Airy was Medical Inspector to the Local Government Board, a government body that oversaw local administration of public health and relief to the poor in England and Wales in the late nineteenth and early twentieth centuries. At the time he wrote, physicians in England and the United States accepted Darwin’s theory of evolution (Zampieri 2009) but the germ theory was still controversial. Airy applied Darwin’s recognition of the importance of variation to disease: “Variation of form or function must carry with it a corresponding variability of the disorder to which that form or function is liable; that is to say, there must be variability of disease.” In particular, he wrote, infectious diseases may exhibit variation in their infectivity. “[I]rrespective of any theory as to the nature of infection, there would appear to be a priori grounds for believing that in any disease in which a tendency to become infectious is at all possible, that tendency is likely to be developed and intensified in course of time… [A disease] which has in any appreciable degree the property of infectiveness, will attack more persons than a non-infectious disease, and those who contract the disease in a more infectious form will be more likely to transmit it to others. In short, the more infectious the disease becomes, the more numerous will be the persons attacked by it, and the greater (ceteris paribus) will be its chance of survival by continued reproduction.”
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Airy appreciated that we evolve in response to our infectious diseases. “In successive generations of mankind there must be, on the whole, a tendency towards insusceptibility of fatal infection, that is, of the particular infections that are current at the time, because those individuals who are the least susceptible of infection are, on the whole, the most likely to leave progeny; and their progeny will probably inherit, in greater or less degree, the parental immunity.” Because humans evolve to be less susceptible to infectious diseases, infectious agents must also evolve. “If the morbific organism itself were incapable of variation, the human race would, by natural selection, become proof against infection by that organism; unless, with susceptibility to infection, there were correlated some contingent advantages relatively more powerful than the risk of a fatal termination of the infectious disease.” Therefore, Airy continued, “the contagium… itself must be capable of variation, to match the variability of its human home and food. As the human syncrasy varies in the direction of insusceptibility, so the parastic microzyme will, step for step, attain new virulence, and… as long as man and microzyme are allowed to meet, I see no possibility of a cessation of the process.” Airy understood that the outcome of infections had different implications for us and for our pathogens: “For man the most serious contingency is that the disease may have a fatal termination: for the microzyme, it is not of vital importance that its human nidus should perish; and it would seem that natural selection might find its equilibrium (though continually fluctuating), and be fairly satisfied with so much variation on both sides as should leave man susceptible of infection, but not in a fatal degree.”
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Airy concludes “that the great law of evolution by natural selection operates in the realm of disease, as well as in health; that infectious diseases are doubly subject to its operation; and that, whatever theory we adopt as to the intimate nature of infection, we cannot hope to know its past history, or anticipate its tendencies in the future, unless we study it in relation to that grand principle which is paramount through all the phenomena of life.”

Airy clearly grasped and articulated the principles of host-pathogen coevolution. Unfortunately, his ideas were neglected and forgotten. Had they received the attention the deserved, he would now be celebrated as one of the founders of evolutionary medicine.
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Bibliography

Airy, H. 1882. On infection considered from a Darwinian point of view. Trans Epidemiol Soc Lond 4:247–61.

Anderson, R. M., and R. M. May. 1991. Infectious diseases of humans: Dynamics and control. Oxford: Oxford University Press.

Fenner, F. 1983. The Florey Lecture, 1983. Biological control, as exemplified by smallpox eradication and myxomatosis. Proc R Soc Lond B Biol Sci 218(1212):259–85.

Zampieri, F. 2009. Medicine, evolution, and natural selection: An historical overview. Q Rev Biol 84(4):333–55.