I was wondering what reasons are that make some viruses become more lethal over time.

By "more lethal", I am not referring to acquiring more mutations which make the virus more infectious and result in more people dying.

I am referring to a drop in survival rate per infected person, because a virus changed in another way than becoming more infectious.

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    $\begingroup$ Welcome to the site. Please take a tour and visit the help center to see what makes a good question. SE biology in particular values questions that make some attempt at indicating that the person asking has done some prior research attempting to answer the question themselves. So - what evidence do you have that some viruses become more lethal and that this occurs not because they are more infectious? $\endgroup$
    – bob1
    Aug 25 at 11:22
  • $\begingroup$ I think implicit to this question is that it is more beneficial for the virus if the host doesn't die. Could just boil down to statistics: More ways to go wrong than right so more likely to not coexist than coexist. $\endgroup$
    – DKNguyen
    Aug 25 at 14:59

It depends on what is implied by over time in the question.

Virus may become more deadly (or otherwise harmful) simply due to a random mutation. However, virus does not specifically aims at harming the host - rather the negative consequences for the host's health are a byproduct of the virus hijacking and killing the host cells. This can be, e.g., due to the toxins generated during the viral replication, or due to the new virions bursting out of a cell and thus destroying it, or because of the overreaction of the host's immune system. E.g., HIV preys at the immune CD4+ cells, whose count eventually drops below the critical level, making the host susceptible to opportunistic diseases.

Thus, in a short run virus becomes more harmful/lethal, as the byproduct of being more successful in replicating and propagating itself. However, in the long run such success harms the virus, since, as an obligate parazite, it cannot exist without a host. Reduced number of hosts means reduced possibilities for virus to replicate. A virus that kills all of its hosts goes extinct. The viruses that continue exist do so either because they have reached an endemic equilibrium with their host or because they can survive in a host of different species, occasionally spilling into human population (like Ebola).

  • $\begingroup$ Sometimes, a virus may indeed benefit from specifically harming the host - a dead host may for example facilitate dispersal of virions. A virus in the short run also may not become more harmful/lethal even if it becomes more successful in replicating. There are multiple levels of "trophism" involved depending on scale - subcellular, cellular, organ-level, whole organism, population, and kinetics may act against reaching an equilibrium state. I think it's important to be specific about scale, time span, etc when asking such a question. $\endgroup$
    – Armand
    Aug 25 at 17:41
  • $\begingroup$ @Armand thanks, these are valuable points. $\endgroup$ Aug 25 at 18:00

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