I've recently been interviewing for grad school, and at one of the institutions I visited, I had a very interesting conversation with a faculty member. We were talking about the evolution of disease in the face of all sorts of selection pressures: drugs, immune responses, other microbes, etc. During the conversation, we turned to talking about coevolution, and this faculty member brought up the point that it seems that the selection pressures involved in the "arms race" between a human host and a disease is fairly one-sided, i.e., our immune system forces our pathogens to adapt (in the evolutionary sense), but most of our immune system "adaptation" happens only at the somatic (physiological) level. By this, I mean that vertebrates have evolved an immune system that has the ability to generate countless numbers of antibodies and the like simply by rearranging and splicing genes within the immune cells. So, any immunity acquired by an individual is not passed along to his/her offspring, and the offspring much start as a completely naive host.
So, my hopefully discussion sparking questions are: Do you think that this is a generally accurate statement? And in terms of human (or more correctly-vertebrate) and pathogen interaction, do you think that we have evolved an adaptation that actually prevents any further evolutionary adaptation to disease?
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