Non-biologist here so apologies if the question is violating too many of the community standards for asking a question in the forum.

What got me thinking was imagining how much more terrifying the current situation would have been if the COVID virus had very long incubation period of something like 6 months and a higher mortality rate with the same rate of spread. By the time someone realizes that a virus is spreading, it would have already infected the entire planet. If it has a very high mortality rate as well, wouldn't that pretty much mean the end of the world?

My question is, what is the probability that this can happen? For example, what is the probability that a virus starts spreading with an incubation period and mortality rate of HIV but has the spread rate of COVID?

  • $\begingroup$ HIV and covid are two very different viruses, which behave diferently in how they are transmitted, and how they replicate themselves within cells. You may want to read this answer, to get a bit more background on how viruses lifestyles differ: biology.stackexchange.com/a/97324/59521 $\endgroup$ – Vadim Dec 22 '20 at 10:08
  • $\begingroup$ Thank you for the response, I understand your point about the HIV virus and how it's very different from the COVID one, and maybe I should have phrased my question differently, but my question was along the lines of, "what is the probability that a virus like the COVID virus mutating so that it has properties like a really long incubation time and more mortality rate" $\endgroup$ – Ananda Dec 22 '20 at 10:15
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    $\begingroup$ Questions of probability are very hard to answer with any amount of confidence. I think a related question that is easier to answer may be: What are the viral traits that underlie measured variables like incubation time and mortality rate? Are there constraints imposed on the trade-off between these traits? $\endgroup$ – acvill Dec 23 '20 at 17:15

HIV and SARS-CoV-2 are two very different viruses in terms of how the are transmitted, how they replicate within the cells, how they interact with the immune system, etc. All of these aspects of a virus existence are highly adapted to assure virus survival (otherwise this virus quickly goes extinct). The ongoing mutations do help a virus to adapt to somewhat changing environment, but do not radically change it.

Thus, a virus that combines a long incubation period of HIV while being highly contagious as SARS-CoV-2, would be a completely new species, different from either of these two viruses in its genetic makeup, lifestyle, etc. It could take millions of years of evolution for such species to develop. Moreover, such a development is unlikely, since such a virus would exterminate its host, thus becoming extinct itself. The scenario that is more plausible is that of Ebola - a virus that exists as relatively harmless in some animals, and can evolve within them, but is highly lethal when it spills to humans, which results in deadly out breaks that quickly end. Probability of such a nightmare scenario is as small as a probability of the world ending due to a black hole generated in Big Hadron Collider or huge meteorite striking the Earth - while it cannot be mathematically excluded, it is too small to account for it in our daily lives (as compared to other dangers).

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    $\begingroup$ That's good to know. Thanks for the explanation. I can sleep a bit better at night. $\endgroup$ – Ananda Dec 22 '20 at 10:47

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