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First, here is what I know:

Some infectious illnesses, such as HIV or Hepatitis B, can be treated with the help of some drugs. The problem is that the virus sometimes undergoes mutations which grant resistance to the drug.

First question : Is it so far right?

Secondly, what is not totally clear is why taking drug does speed up the development of the drug-resistant version of the virus. In my point of view, the drug resistant virus will develop anyway with or without taking drugs. I don't see what taking drugs changes. Is it like an intra-competition between the different mutated versions of the virus and in this case, taking drug will eliminate all the mutated versions that are not drug-resistant and, this way, favour the development of the drug-resistant versions?

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  • $\begingroup$ Welcome to Bio.SE! your both assertions are quite correct. yes, mutations are random and it is the drug itself which favours evolution of drug-resistant strains by killing non-resistant ones. $\endgroup$ Commented Feb 6, 2017 at 12:00
  • $\begingroup$ Ok, thanks! If I understand well, there is a competition between all the forms, versions of the virus? If yes, why cannot they develop together? Is that because they need the same resources to develop? $\endgroup$
    – Anthony
    Commented Feb 6, 2017 at 12:10
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    $\begingroup$ I am no expert in evolution, so I can't give you very precise answers. The point is that without the drug, both resistant and nonresistant strains can live happily without competition (what would they compete for?) but when drug comes in, it kills nonresistant ones, leaving resistant ones able to dominate. $\endgroup$ Commented Feb 6, 2017 at 12:15
  • $\begingroup$ My central question is : why does taking drugs favour the development of drug-resistant strains? If there was no competition between strains, taking drugs would have no effect on the development of drug-resistant strains, and it's not the case. So, this is why I suggest a kind of competition between strains. Do you understand my reasoning? $\endgroup$
    – Anthony
    Commented Feb 6, 2017 at 12:47
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    $\begingroup$ No, they mutate at a similar rate; taking the drug does not affect mutation rate, but the idea of drugs is to inhibit the virus somehow. This happens in nature all the time (an organism develops - through a mutation - a product which can inhibit growth of a competitor. Taking the drug provides a selective advantage for the drug resistant 'organism' (or virus) to proliferate, as @another'Homosapien' has already stated. $\endgroup$ Commented Feb 6, 2017 at 14:53

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