How does a micro-organism causing some communicable disease infect the first organism it infected?

I was reading about HIV, when I found that HIV has jumped from chimpanzees to human beings. But, how did the first chimpanzee affected by HIV get affected?

In other words, is it possible (without artificial means) for a new infection to get started? How?

NOTE: I am not aksing about the origin of pathogens, rather, about how they infect the first organism infected from it, and thus, become a disease causing microorganism.

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    $\begingroup$ Does this answer your question? How did viruses come to be? $\endgroup$
    – tyersome
    Dec 14, 2021 at 21:16
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    $\begingroup$ Another related question. Please note that you are expected to do prior research (including, but not limited to searching on this site) before posting. In addition, micro-organisms includes many pathogens that are not viruses, so your question is unfocused and poorly worded as well as under researched. Please take the tour and consult the help center starting with How to Ask for details about what is expected. $\endgroup$
    – tyersome
    Dec 14, 2021 at 21:26
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    $\begingroup$ @tyersome sorry, but I didn't mean only viruses, its just while reading about HIV, this question came in my mind $\endgroup$ Dec 15, 2021 at 2:30
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    $\begingroup$ and @tyersome, I did not want to know how viruses or other microorganisms are "formed", instead, my focus was on how it "infects" the first organism it infected. For example, Rhinovirus (causes common cold) may have formed from dna fragmemts, or whatever. I was asking that how did the first rhinovirus-infected organism get infected? $\endgroup$ Dec 15, 2021 at 2:33

2 Answers 2


Viruses evolve like all other biological species. The family of SIVs that gave rise to HIV goes back millions of years across many hosts.

Just like you can build a "family tree" out of related plant or insect or mammal or bacterial species, you can do the same with viruses. The family tree of HIV and SIV goes back to a broader classification of lentiviruses that includes some affecting non-primate mammals.

Viruses affect all types of life: bacteria and animal, plant and protist. They've likely existed as long or nearly as long as cellular life.

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    $\begingroup$ Oversimplified, your answer means new infectious diseases are caused by evolution of already existing micro-organisms? $\endgroup$ Dec 14, 2021 at 18:30
  • $\begingroup$ If that's true, then I got it, thanks. $\endgroup$ Dec 14, 2021 at 18:30
  • $\begingroup$ @AsmitKarmakar Yes, that's accurate. $\endgroup$
    – Bryan Krause
    Dec 14, 2021 at 18:37

The key term to find information pertinent to this question is Zoonosis (link to the Wikipedia article).

However, one should not generalize zoonotic origin to all the diseases affecting humans - in fact, the relationship between humans, diseases, and other organisms carrying these diseases is rather complex.

  • Some diseases are uniquely human: their origin is unknown - they might have evolved together with humans or were transmitted to humans or their predecessors millions or billions years ago. Smallpox is one example - it became eradicated as soon as it was eradicated in human population.
  • Other diseases have evolved to be transmitted between different types of hosts, e.g., such as humans and mosquitoes. Flaviviruses, such as Zika, Yellow fever, dengue, west Nile virus are examples of such pathogens, adapted to two different types of hosts. In fact, some of these are adapted to mammals other than humans, but capable of infecting humans due to sufficient similarity. Some of these might not be able to effetcively replicate in humans, even though being a serious health hazard.
  • For some diseases insects (or other organisms) serve as mechanical vectors, merely carrying disease from one human to another or from a human-like animal to humans, while not infecting the vector.
  • There are animals that can be infected by both animal and human viruses. E.g., the types of influenza affecting many birds are not dangerous for humans, and these birds do not get infected with human influenza. However, pigs and paultry may get infected by both. The influenza viruses coexisting in a pig then undergo gene reshuffling, producing new viral strains infectious to humans, that humans get from pigs.
  • Finally, what the OP probably had in mind is the transmission of viruses such as Ebola, SARS/MERS, and HIV which exist in nature as animal viruses and suddenly transform to become human viruses. This is an evolutionary process, which is rarely successful.

E.g., the bushmeat consumption has probably on many occasions introduced SIV to human population - perhaps thousands or millions times during the history of this virus. However, not being adapted to humans, the virus didn't cause pandemics - until the moment when this transmission as accompanied by a fortunate mutation (or mutations) that allowed it successfully replicated and be transmitted in human population. It is quite possible that it took some time for this virus to further adapt to human population before it became as lethal as it is.

Ebola is an example of virus that seems to be poorly adapted to human population - although successful in replicating itself, it fails to keep enough hosts alive to assure its permanent existence in the population. Last century has seen multiple Ebola epidemics, all successfully contained by quarantine measures. There have been probably many more spillovers over the course of human history, but they all ended with virus becoming extinct in the human population by eliminating available hosts.

  • $\begingroup$ Can you tell me how: 1) Uniquely human diseases got into the human population? 2) Multi-host disease causing pathogens infect the first host it infected? 3) Pathogens which use insects as vectors infect the first human it infected? 4) Pig influenza virus get into the first pig it infected? 5) HIV/ Ebola viruses (before mutation) infect the first animal they infected? $\endgroup$ Dec 16, 2021 at 6:11
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    $\begingroup$ The two threads linked (here and here) have some information, but no one knows for sure, since it happened long time ago. HIV, Ebola, Zika, SARS/MERS are well studied, since they happened recently. Some other disiseases are merely mentioned in historical records from a thousand or two thousand years ago (see also this thread), but we cannot look back any further than that. Unlike dinosaurs, viruses and bacteria have not left fossil records. $\endgroup$
    – Roger V.
    Dec 16, 2021 at 8:02
  • $\begingroup$ okay.. thanks:) $\endgroup$ Dec 16, 2021 at 8:07

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