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From my basic understanding: The viruses causing Ebola, Sars and Covid-19 are all the result of a zoonosis, meanings that the viruses have passed from animals to humans.

So my question is: Are all recently (let's say 100 years) emerged viral diseases, with potential for a global epidemic, the result of a zoonosis?

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    $\begingroup$ At least a lot of them. I don't like linking to Wikipedia for such matters, but they have a nice list of sybtypes of Influenza A that should give you an example of how many variations you can have on the same base. From H1N1 to H5N1 and beyond, some of those variations were endemic, some caused pandemics. Some of those that stayed endemic had the potential to become pandemic. $\endgroup$ – Mast Mar 16 '20 at 15:24
  • $\begingroup$ In general, viruses don't profit much from their host dying or becoming too ill to infect others. Viruses that "rely" on humans to procreate are often not lethal - the lethal variants of the virus have a lower propensity to spread, so the less lethal ones are "fitter". Zoonoses have not been culled to be less lethal to humans, and are often much worse for us. We are "dead-end hosts" to them, We are best killed off so the viruses can spread to the preferred host, in laymans terms. Fortunately, we have murdered most mammals larger than us, so we are safer now than before. $\endgroup$ – Stian Yttervik Mar 18 '20 at 9:39
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    $\begingroup$ @StianYttervik, this is a somewhat naive/narrow view of virulence evolution. Smallpox circulated in the human population for many thousands of years with a case fatality rate of 30% ... $\endgroup$ – Ben Bolker Mar 18 '20 at 13:58
  • $\begingroup$ @BenBolker There are certainly exeptions, I am making a general case. And, afaik (and googled right now) smallpox was a zoonoses, and thus is not a contrary observation. It's lethality was supported because it had an alternate (preferred host). $\endgroup$ – Stian Yttervik Mar 18 '20 at 14:05
  • $\begingroup$ Basically everything is a zoonosis at some point. I'm not aware that smallpox had a reservoir host: what was it? biology.stackexchange.com/questions/90636/… gives starting points for the modern literature on evolution of virulence ... $\endgroup$ – Ben Bolker Mar 18 '20 at 14:07
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To my knowledge, yes. A partial list of recently emerged/emerging viral diseases (I certainly could have missed some), with probable reservoir hosts:

Starred examples are vector-borne (so perhaps of slightly lower concern - might not fit your criterion of "capable of causing a global pandemic").

Omitted:

  • older zoonotic viruses (rabies, dengue, hepatitis, ...)
  • non-viral zoonoses (malaria, plague, anthrax)

A list of zoonoses; another from US CDC

More generally, the only other place an emerging virus could come from would be from mutation or recombination of existing human viruses. I'm not aware of such an example.

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    $\begingroup$ Corrections and additions welcome (I could make this community-wiki) $\endgroup$ – Ben Bolker Mar 15 '20 at 20:48
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    $\begingroup$ there are human viruses, it is just a virus that evolves with humans is not going to be that destructive, killing your host is always a bad idea and viruses evolve away from it. ncbi.nlm.nih.gov/pubmed/14666532 $\endgroup$ – John Mar 16 '20 at 0:45
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    $\begingroup$ @John evolving away from destructiveness includes cases where a virus evolved into something destructive anyway, but died out along with the population segment that was affected by it. So while destructiveness doesn't persist, there's always the possibility of a short-lived outbreak. $\endgroup$ – toolforger Mar 16 '20 at 6:35
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    $\begingroup$ In the case of monkeypox, there was a 2003 outbreak traced to pet prairie dogs, who were themselves infected by African rodents (two African giant pouched rats, nine dormice, and three rope squirrels were found to be infected), suggesting rodents are a likely reservoir. $\endgroup$ – Jacob C. Mar 16 '20 at 22:51
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    $\begingroup$ @Mast: are there recently emerged strains of hepatitis? A very quick search suggested that at least hepB elifesciences.org/articles/36709 and hepC journals.plos.org/plosone/article?id=10.1371/… are pretty old. Pointers to papers on emergence in the last century? $\endgroup$ – Ben Bolker Mar 17 '20 at 0:16
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Hepatitis D emerged in the past 100 years, without being a zoonosis

Hepatitis D is a virus which is able to replicate only in the presence of a hepatitis B co-infection. It causes the same symptoms as the hepatitis B virus, but with greater severity and lethality. In developed countries, it is rare except among intravenous drug users.

It was discovered in 1977, although epidemics of Lábrea fever in the 1950s were later determined to be caused by the virus. Comparisons of known strains and the mutation rate suggest a common ancestor around the year 1930.

Hepatitis D is the smallest known animal virus (less than 1700 nucleotides), and is the only species in the Deltavirus genus. There is no known animal reservoir. The most similar known animal virus shares only 32% of the nucleotides of hepatitis D.

According to its Wikipedia page,

It has been proposed that HDV may have originated from a class of plant pathogens called viroids, which are much smaller than viruses.

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    $\begingroup$ Even if indeed this virus originated in plants, is it really true that humans were the initial animal target? That seems exceedingly unlikely, and there are other similar viruses that infect other animals as well. $\endgroup$ – Bryan Krause♦ Mar 17 '20 at 14:04

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