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This question already has an answer here:

An ideal parasite is a parasite which thrives within a host without harming it (according to my NCERT textbook).

Then, quite obviously, why hasn't that happened yet? I mean, natural selection can lead to such a species, which is beneficial to us and also, not at all detrimental to the parasite.

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marked as duplicate by Remi.b, AliceD, rg255, MattDMo, Chris Mar 31 '16 at 5:25

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    $\begingroup$ It has happened, many times; commensal and mutualistic species are common. Mitochondria and chloroplasts, lichens, etc are examples. But there is no specific reason, other than common misunderstandings of natural selection, to expect this to be inevitable. You may want to explain your reasoning for that point (or put it as a separate question). $\endgroup$ – iayork Mar 28 '16 at 18:19
  • $\begingroup$ @iayork You are right. However, how are mitochondria and chloroplasts examples of ideal parasite? Also, which point are you asking me to explain? $\endgroup$ – ajitanshu singh Mar 28 '16 at 18:23
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    $\begingroup$ Related: Why do parasites sometimes kill their hosts? $\endgroup$ – Remi.b Mar 29 '16 at 4:44
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    $\begingroup$ There was a recent opinion article in science which hypothesizes that mitochondria and chloroplasts could have been parasites. In my opinion, the paper has a lot of incorrect presuppositions and also makes far-fetched assumptions. IMO, mitochondria and chloroplasts would have been preys and not parasites. Anyway, an ideal parasite is the one that survives anything. What your textbook says is one possible way by which it can do that. You should see the post that @Remi.b mentioned. $\endgroup$ – WYSIWYG Mar 29 '16 at 5:59
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    $\begingroup$ Post is related to Why do parasites sometimes kill their hosts? and I think duplicate of Why have parasites not evolved to be harmless?. $\endgroup$ – Remi.b Mar 30 '16 at 17:03