The theory and philosophy is basically an evolutionary arms race - adaption to conquer other organisms with the subject counteracting that very adaption, or we can say, "Keep evolving (the Red Queen effect) or else be doomed." In theory if a host does not adapt to the infection, the pathogen will eliminate it altogether. Then, there is no net benefit for the pathogen either. If the host adapt too much say, it is again no good for the pathogen.

You may try these accessible articles:

Article On the principle of host evolution in host–pathogen interactions

Article Horizontal Transfer and the Evolution of Host-Pathogen Interactions

Article The evolutionary conundrum of pathogen mimicry

Article Hereditary red cell disorders and malaria resistance

http://www.aecom.yu.edu/uploadedFiles/casadevall/casadevallevol.pdf

http://www.thereadgroup.net/wp-content/uploads/Read_94.pdf

http://www.academia.edu/download/32329191/_78-ARG_rulesofengagement.pdf

This "Reciprocal coevolution" is fairly common, even in humans as well.

A well-studied example is that of sickle cell anemia. In regions where malaria is endemic, the red blood cells have acquired a "sickle" shape, rather than the "biconcave disc" shape through genetic changes (substitution of valine by glutamic acid in the 6th position of the beta globin chain). This sickling effect (though has its own problems) made the cells immune to the plasmodium parasite and thus protected the individuals from acquiring malaria! However, with time the plasmodia began to have their own genetic alterations and now there are strains able to infect sickled cells as well.

A few Darwinian principles are based on this concept as well.