All organisms are under attack. If they respond well enough to go on to reproduce, they win. There is a great variety of ways that organisms respond, from simply repro ducing soon and often, to having very complex, staged molecular machinery to identify, isolate, and destroy pathogens at the molecular level and, in some organisms, to remember what happened earlier so that in the event of subsequent exposure the invader can be destroyed more quickly. The organism might also just tolerate or become commensal with the pathogen or may out-reproduce or simply endure it.

It is too easy to reconstruct "had to" stories for these systems. Except for the most primitive and nonspecific innate systems, there is clearly no mandate to have an immune system of any particular type or efficiency. Either the more complex systems overlaid on simpler systems initially (and perhaps still) serve some other function, or the circumstances that called for them no longer exist, or we do not understand them. The complexity of immune systems today is "good enough" but not perfect or of the same type in different species. Slow metabolism or rapid reproductive strategies, or reproduction from diverse sequestered parts (as in plants), may allow an organism to tolerate infection rather than having to fight it.

It is not hard to see how molecular defense strategies might evolve. Any ligand-binding system in an organism might be modified by mutation (perhaps after gene duplication) to produce alleles that bind things not normally part of the system. If it binds the wrong things too aggressively it is selected against (and we might call that a "disease"), but if it does not bind self-structures too aggressively it may end up binding to other things that get inside the organism, inactivating them, and being favored by selection. It can then undergo evolutionary elaboration.

A subject beyond this book is the ecological evolution by which species adapt to commensalisms or tolerance. From a selfish evolutionary point of view, a pathogen that kills its host can survive only if transmission to a new host occurs quickly and readily enough (that is, before the original host is dead and gone). The conditions for epidemic versus endemic infection are debated among immunologists and epidemiologists (Ewald 1994). Many parasites kill their hosts only slowly or if the latter are under nutritional or other stresses. They are endemic because they are always there to infect new susceptibles born or migrating into the population. Other parasites cause little or no reaction by the host and do no harm. As we have noted above, some parasites are symbiotes: we cannot do without them, and they cannot do without us. Each case is different. The important point is that immune systems and/or parasites evolve so that the immune reaction is strong enough to protect the host but not necessarily any stronger. Of course, neither the strategy of host nor that of the pathogen is always successful. Those that have overstepped their bounds are generally no longer here to be studied.

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