Why Offense Is Not Always the Best Defense
One of the triumphs of 20th century immunology was the documentation in exquisite detail of the mechanisms animals have for killing infectious disease agents.
Vaccination demonstrates that these mechanisms can be highly effective. Yet attack will not always work. Pathogens are frequently very slippery targets, with host–parasite coevolution generating a bewildering array of immuno-evasive or immuno-suppressive strategies.
The organisms we recognize as pathogens are, by definition, staying ahead in host–pathogen arms races.
This is partly because they generally have much shorter generation times than their hosts, but also because the fitness consequence of resistance is normally much more severe for pathogens than is the fitness consequence of infection for hosts.
In antagonistic interactions, hosts are therefore doomed at best to ever-transient partial success.
A second reason why attack is not always the best defense is that killing infectious agents can be very costly [5].