As in human warfare, attack can result in considerable self-inflicted collateral damage, and/or vast outlays in resources. Consequently, natural selection will, in many circumstances, favour the evolution of protective mechanisms that do not involve pathogen killing.
It may even be that the majority of host defense mechanisms that have arisen during evolution are tolerance mechanisms.
For one thing, natural selection is more likely to drive alleles conferring tolerance to fixation (reaching 100% frequency in a population) [6].
In contrast, resistance mechanisms work by eliminating parasites, and thus undermine the very selection pressures that favoured them in the first place.
As a particular resistance mechanism nears fixation in a host population, parasites must change or die out, rendering the resistance mechanism unnecessary or useless.
Tolerance will not prompt antagonistic counter-adaptation by pathogens, since it does not harm pathogen fitness [2].