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.

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    $\begingroup$ You seem to assume that the virus does not evolve or maybe you mean that it is selected for hurting its host (which is wrong). It is quite unclear what you mean by "evolutionary purpose" as well. You might take advantage of following a short intro to evolutionary biology such as Understanding Evolution for example. You might also want to have a look at Why do parasites sometimes kill their hosts?. $\endgroup$
    – Remi.b
    Commented Oct 20, 2016 at 18:25
  • $\begingroup$ This is a bit like asking what the evolutionary value is of developing cancer in only one lung. There is none. Cancer is cancer; it has to start somewhere. Shingles is a physiological process involving viral reactivation in one dorsal root ganglion. Why should it be bilateral? You're looking at this from a teleological standpoint, which is very often a mistake in science. $\endgroup$ Commented Oct 20, 2016 at 20:06
  • $\begingroup$ Well since the chicken pox virus (hhv 3) that also causes shingles, practically spares no one, is it not already successful? $\endgroup$
    – Polisetty
    Commented Oct 20, 2016 at 21:54
  • $\begingroup$ @Polisetty Good point and similar to one in the accepted as answer below. @ anongoodnurse Cancer is the cell division system gone haywire (even if triggered possibly by a virus such as HPV?), whereas a virus is not intrinsic to its host? $\endgroup$
    – armipunk
    Commented Oct 21, 2016 at 14:11

1 Answer 1


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.

  • $\begingroup$ The virus' replication and its own spread makes sense. I was looking at a greater degree of symbiosis, if you will, between parasite and host. Unlike the common cold virus which typically has non-fatal affects on humans and hard to say surviving a makes one stronger than another. I think you can make that argument that only people with stronger immune systems survive the small pox virus. I am still puzzled by the chicken pox virus - non-fatal in kids, hibernates for 25-30 years before re-emerging - why? Why this cicada-style lagged cycle? $\endgroup$
    – armipunk
    Commented Oct 21, 2016 at 14:33
  • $\begingroup$ While symbiosis is possible between "parasite and host" it isn't necessary; many parasite/host relationships are not at all symbiotic. It isn't correct to say that a virus that kills people with weaker immune systems is symbiotic by "improving" the immune systems of the population of hosts. The hibernation is a way for the virus to avoid the host's immune system while that immune system is most prepared to hunt it down; it doesn't need to have any benefit to the host. $\endgroup$
    – Bryan Krause
    Commented Oct 21, 2016 at 15:01
  • $\begingroup$ Understood - a consequence of a small pox epidemic be people with better immune systems but that 'gain' is not necessarily obtained via a symbiotic relationship. $\endgroup$
    – armipunk
    Commented Oct 21, 2016 at 16:20
  • $\begingroup$ Yeah, I'm not certain that treating that as a 'gain' is really appropriate - stronger immune systems might also lead to autoimmune disorders, for example. I think it is safer to think of it as an evolutionary adaptation: animals including people evolve defenses against viruses of all types because those viruses cause harm if not kept in check. But that is a result of the evolutionary process, not a "reason" or "purpose" for viruses to exist. $\endgroup$
    – Bryan Krause
    Commented Oct 21, 2016 at 16:36
  • $\begingroup$ @Bryan Krause Symbiosis is an interaction between two organisms. May be parasitism, mutualism or commensalism. Although its just semantics, still, its a consensus. en.m.wikipedia.org/wiki/Symbiosis so all parasites are in a symbiotic relation with the host $\endgroup$
    – Polisetty
    Commented Oct 22, 2016 at 4:31

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