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Why rabies virus has nearly 100% fatality rate in human (see this virology blog; thanks to @iayork for the link) if not vaccinated early, even some people have survived Ebola, then why does rabies doe not get extinct as its most virulent form will kill host and virus also?

Remi.b
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murmansk
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    Would be good to have a reference for the claim rabies virus has 100% kill rate in human if not vaccinated early – Remi.b Dec 07 '17 at 19:18
  • @Remi.b http://www.who.int/mediacentre/factsheets/fs099/en/ – murmansk Dec 07 '17 at 19:52
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    @murmansk Maybe I looked poorly but I failed to find the claim you are making. Sorry to be a pain but could you highlight where the claim is being made? – Remi.b Dec 07 '17 at 19:55
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    Speaking as a virologist and veterinarian it's absolutely true that the mortality for human rabies approaches 100%. How lethal is rabies virus?, List of human disease case fatality rates, Rabies - Prognosis – iayork Dec 07 '17 at 19:59
  • @Charles http://www.who.int/rabies/resources/SEA_CD_278_FAQs_Rabies.pdf question no 11 – murmansk Dec 07 '17 at 20:02
  • @Charles there are a half-dozen known survivors, but considering that the virus kills over 50,000 people every year, bloviating about the difference between 100% and 99.9999999% seems pretty silly. – iayork Dec 07 '17 at 20:05
  • @iayork This is quantitative science. Please do not try to argue that it's "silly" to distinguish the difference between 99.999% of the time, and 100% of the time. Depending on the system/behavior in consideration, this can have absolutely profound implications. –  Dec 07 '17 at 20:07
  • @Remi.b if you want to mark this as a duplicate for the question you answered, I'd suggest significantly improving your answer there; it completely ignores a very large literature on pathogen selection. – iayork Dec 07 '17 at 20:13
  • @Charles I've now edited OP's post. – Remi.b Dec 07 '17 at 20:18
  • @Remi.b I noticed :) I've removed previous comments, and have reversed my vote. –  Dec 07 '17 at 20:19
  • @iayork If the current answer at the other is not fully satisfactory, then a new post should address the reason why it is not fully satisfactory, not just ask the same question over again. I would not mind if the OP wants to argue that his post is specific to rabies and it is interesting because... but it should be clarified then IMHO – Remi.b Dec 07 '17 at 20:19
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    @Remi.b There are already better answers in Why have parasites not evolved to be harmless? – iayork Dec 07 '17 at 20:25

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It's a common misconception that pathogens "don't want to kill their hosts". In reality, pathogens don't "care" at all about killing their hosts. All pathogens "want" to do is transmit from one host to the next. (Taking out the teleogenic language: There's no natural selection on a pathogen to reduce virulence as such. The selection on a pathogen is for transmission.)

In some cases, transmission is optimized when the virus is less virulent. In other cases, transmission works just fine when the virus is highly virulent. Rabies is one of the latter cases.

If a pathogen is spread by, say, respiratory contact, then having a host that is ambulatory and seemingly healthy may allow the first host to contact many others, so reducing virulence may enhance transmission in that specific case.

If the pathogen is spread by sand fleas (as in myxoma in Australian rabbits), then rapidly killing the host is counterproductive, because fleas leave a dead body. But keeping the host healthy is also counterproductive because a healthy rabbit scratches away its fleas. Optimal transmission turned out to work with a virus that kills 50-75% of its victims, because that makes the host very sick for a long time, allowing the fleas to bite it unimpeded.

Rabies spreads by damaging the brain, making the host aggressive, and spreading through bites in saliva (among other ways, but that's an important one). Serious brain damage to the point of wildly abnormal behavior is really not compatible with a host living a long time, so optimal rabies transmission means very high host mortality.

Finally, humans are not natural hosts for rabies and rarely transmit the virus, so in any case there's minimal natural selection for any virulence changes in humans. But it likely wouldn't matter, for the reasons above.

iayork
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