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It's bothered me for a while now. I understand why a parasitoid would do this, as it only temporarily requires the host, and that not all parasites kill their hosts.

There seems to be no evolutionary advantage in killing a host, because the parasite relies on the host for resources. Yet some organisms, usually microscopic pathogens, seem to fatally damage the host with no immediate benefit to themselves. Why is this?

I know this is asking for a broad soft answer, but I don't want the question to get bogged down by a specific species, although bonus points for using examples.

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Great question. I wonder if metapopulation dynamics could have something to bring to this conversation. That is, it can be appropriate to exhaust your resources in one "patch" (eg., glade, island, host organism) if it's relatively easy for you or your offspring to move to another "patch" –  Oreotrephes Jul 26 '13 at 7:53
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3 Answers

up vote 9 down vote accepted

I can think of several (non-exclusive and non-exhaustive) hypotheses:

  1. It is maladaptive. Because it is a new virus coming from another species or because it is not adapted to our modern lifespan.

  2. Some might benefit from host immune defense (sneezing helps bacteria to spread) and death is a consequence of the selection for increasing disease symptomes to the host.

  3. The pathogen's fate is not linked to the host's fate. This is espacially true if the pathogens spread well (easily jump from one host to another) I guess or for parasitoïd because the pathogen leave its parasitic life after the death of its single host.

  4. Between hosts, there might have a selection for decreasing the probability for the host to die. But within a host, it is a prisoner's dilemna (tragedy of the commons). The more you invest into foraging, the more competitive you are. This might not be adaptative at the population level but it is at the individual level. We might also think of this prisoner's dilemna by including several pathogen species

  5. Harming an individual will cause him not to spend energy into reproduction in order that more energy is available for the pathogen use. Moreover, by causing symptoms, the infected individual will be sexually less attractive and will again have less opportunity to spend energy into reproduction.

There's a book written by Randolph Nesse and George Williams called Why we get sick that might interests you.

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#3 is a common answer, and linked to #2,4,5. Lots of viruses for instance simply reproduce and spread to other hosts. If a host dies it doesn't hurt the pathogen if it's timing is good. –  shigeta Jul 26 '13 at 14:08
I like #3, but what exactly do you mean by well spread? Are you talking about method of infecting new hosts, or general population? –  Good Gravy Nov 4 '13 at 12:59
@GoodGravy I meant "if pathogens have good dispersal abilities (good abilities to jump from one host to another), then it is likely that the survival of the parasite is not linked to the survival of the host". Does it make sense? I don't really understand your question above. But I said "spread well" not "well spread". Maybe here comes the doubt. –  Remi.b Jan 7 at 9:23
@Remi.b "Spread well" makes more sense. I misread it! I was asking if you meant dispersal mechanism from host to host, or the ability to infect other species of host. But you have already answered the question. Thanks. –  Good Gravy Jan 7 at 17:26
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Some parasites aren't natural hosts of humans, instead we are an accidental host in which they cannot replicate. Zoonoses such as this can be quite fatal as the parasite is not adapted to us. Although viruses are often separated, they are parasites and you can see just in influenza that the animal derived flus are more fatal than those that are well adapted to humans.

Otherwise because there is no need for the host they can be fatal. If they have finished their round of replication or require death of the host to be able to spread. Still this is less common than simply a badly adapted parasite.

HPV causes cervical cancers. It is adapted to increase replication of its host cell which is obviously beneficial, but this also kills the host in the long term. Thus some viruses are not yet adapted to the new longer lives we live, but may never do so due to less of a survival pressure hence vaccines.

Please feel free to ask questions.

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+1 - a number of the parasites that cause human disease aren't supposed to be in us. We're an odd side-note in their lifecycle. –  Fomite Jan 7 at 20:01
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This is related to Remi.b's answer, but a common reason is that sometimes parasites/pathogens actually need to kill the host to spread. A lot of viruses, in particular phages, are only released from their host by bursting out of the cell. If the parasite can't leave the host, there's no point in any of it, and death may be a good way to achieve that. A number of diarrheal diseases in humans cause death because massive diarrhea (aka water loss) is how they spread.

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A virus usually destroys its host cell to get released (this cell has been reprogrammed by the virus to produce only viral products anyway) but this does not necessarily mean the death of the host. –  Chris Jan 7 at 7:30
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