A question here asks for the name of the category of viruses that affect only one side of the body. My question is about the evolutionary purpose of that 'affects only one side' behavior.
Chicken pox is not usually fatal, and it's not clear to me why evolution would set such a low bar for eliminating the weaklings in the herd. On the opposite end, small pox was excessively fatal; I guess that's the price of evolution biohacking its way by trial and error.
On the other hand, I can see how shingles, afflicting people only in later life, can be debilitating enough to keep one from going hunting and gathering and moaning one's way to death by starvation. But for that to happen isn't it more efficient for the virus to resurface across the entire nervous system instead of parts of just one side? If shingles happened, say, in the late teens, I could understand that 'affects-only-one-side' eliminates only the weaker part of the herd. But it's mostly older people, well past their reproductive prime, who get shingles, and get it only on one side. I see no benefit to the individual or the group of contracting an 'expire by date' virus early in life and then the virus goes about its expiration-dance in a half-hearted, tentative way. Makes no sense.
The premise of your question is wrong. Although evolution is often a good guide when looking for "purpose" in biology, sometimes there are other proximate causes.
Shingles is caused by activity in a previously dormant version of the virus that causes chickenpox. Essentially, your immune system clears out the virus from everywhere in the body except some that hides in the nerve roots.
Peripheral nerves to the skin are arranged into "dermatomes" which are essentially the group of nerves that enter the spinal cord at a particular vertebra.
Shingles happens when the virus becomes active again. Since a dermatome only covers one side of the body, shingles usually only affects that side of the body (in particular, that dermatome).
http://www.webmd.com/skin-problems-and-treatments/shingles/shingles-cause looks like a decent reference for this information.
I don't personally know of research on the evolution of varicella zoster, but if you want to think up a speculative evolutionary process, you should be thinking about the virus and spreading of the virus, which is the activity that natural selection will support. In that case, lying dormant can give the virus an opportunity to spread again at a later date, preventing it from being completely eliminated by the host's immune system. All the business about rashes on one side of the body comes about by 'accident,' as a matter of convenience since the virus hides out in peripheral nerve roots.