One factor that makes it hard for pathogens and hosts to evolve\"perfect\" attacks or defenses is that natural selection acts onexisting systems, which means that altering a system to fit aparticular time and situation may make it less fit in othersituations. A prime example of this is:
> VSG switching.
> All of the above.
> Somatic Recombination vs. Somatic Hypermutation.
> The CCR5 gene and the CCR5-delta32 mutation case.
