What you will actually find is that pathogens are generally pretty bad at what they do, and nature selects against them because they tend to kill their host too

What you will actually find is that pathogens are generally pretty bad at what they do, and nature selects against them because they tend to kill their host too quickly. There are many organisms that infect but do not cause symptoms that propagate much more genetic material that pathogens. Similarly, we don't want to be too defensive. It's expensive, and the more complex our immune systems become, the greater the chance of autoimmune disorders. The more energy we spend protecting ourselves, and not growing. Therefore we will never develop perfect immune systems, and pathogens will never be perfect killers. The arms race analogy does a poor job of illustrating that in actuality, host-pathogen interactions are something of a dynamic equilibrium at this point.