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redligand t1_j4wfxfr wrote

It's very crudely similar to the reason you can't get an organ transplant from your dog despite sharing the same organs. There are certain molecules (mainly proteins) that exist on the surface of animal cells that allow other species to recognise them as foreign. By and large the same kinds of proteins (we say they are homologous) exist on the surface of your dogs cells and yours but they are sufficiently different that your body will recognise them as "not mine".

Viruses exploit their hosts by binding to proteins on the surface of cells. Often these proteins are sufficiently different between species that a virus cannot recognise "Protein X" in a human, even though it operates by binding to the homologous 'Protein X" in a dog. However, it is certainly possible for viruses to evolve to recognise both dog and human Protein X. This can happen most easily when humans and populations of sick animals live closely together for long periods of time. It's one of the leading hypotheses of the emergence of covid, HIV and ebola (to name a few high profile but far from comprehensive examples).

The viruses don't do this on purpose of course. They're mutating randomly all the time. But if a few virions by chance develop a mutation that makes them less selective wrt the differences between Protein X across species AND they have the opportunity to spread to that new species, they can jump the "species barrier" and become a new infectious agent in a new set of hosts.


cheekychessie OP t1_j4wittd wrote

Wow first answer is a champion!! So detailed and really makes sense. Thanks so much.

Follow up question, for something like covid. Was it just waiting to find a human with a similar enough protein match to make the jump?


venom121212 t1_j4wrp6e wrote

It's more like covid evolved in its host species to a form where it was able to affect humans.

To oversimplify things:

Picture a virus as a really funky key. Your cells (animal cells) are covered in funky looking locks. These locks are meant to allow good things in while generally keeping bad stuff out, all throughout your body. When a virus mutates, it is finding a way to spread more efficiently and effectively. One of these methods is by changing binding protein shape to fit more locks.

Similar situation with swine flu, avian flu, and many other zoetic viruses. We keep an eye on which animal viruses are getting dangerously close to making the human jump by taking animal cell lines, replacing the receptors (locks from earlier) with human versions, and then seeing if the virus is able to bind and replicate.


Boring_Ad_3065 t1_j4wxbyf wrote

Adding on to this, some viruses are really good at jumping species. Influenza is one example, hence why you hear of swine flu and avian flu, but different variants can infect seals, cattle, dogs, horses, etc. It doesn’t help that humans raise billions of pigs and chickens for food that are kept in very close, not great conditions, so there are many opportunities to interact with large populations of sick animals. The fear of spreading is why you hear of culls of millions of animals sometimes. This also happened with minks and COVID.

HIV descended from SIV, and was most plausibly introduced to humans when a hunter was harvesting bushmeat, a bloody process, and cut themselves.

It’s important to note that just because viruses can cross species, they can be more or less deadly and transmissible. Typically early versions of viruses aren’t well adapted to new hosts. COVID is likely harmless in bats (who carry many viruses), and clearly deadly in humans, while being poorly adapted to dogs. For COVID, this is due primarily with how our respective ACE receptors are. Additionally even within roughly similar viruses there’s variance. MERS, another coronavirus carried by camels is much deadlier than COVID at ~37% of cases but thankfully far, far harder to transmit.

Finally it’s not all bad for humans. We eradicated smallpox in part because someone noticed that milkmaids were practically immune to it. Cowpox is similar enough that it conferred strong immunity to smallpox, while being much more mild.

A caveat - evolution in general, but for viruses in particular doesn’t “seek” anything. Viruses don’t even meet all typical criteria for being alive, and their evolution is purely a numbers game, with a single infection generating upwards of trillions of viruses, and then “hoping” it comes in contact with another suitable host. Viruses are also messy, and some can share genes very readily. Influenza is a prime example.


cheekychessie OP t1_j4wynrr wrote

That makes a lot of sense and adds context, thank you!!

I’ve actually also always wondered why humans are not so genetically diverse compared to other species. In this context there’s an answer that makes sense to me. A degree of less genetic diversity, compared to a lot of other species, keeps us safer as we have less chances to provide this kind of bridge that viruses need to evolve into something we can catch.

Do you think that’s somewhat right?


HelloRickyHere t1_j4y5z68 wrote

I think mostly it's the opposite of what you describe. Diversity is key here, because if everyone had all those same molecular markers, we'd all be susceptible to the same virus. Mixing things up gives the species as a whole a better chance to survive a bad pandemic, if there are some groups that will be relatively safer (because their cells are harder for the virus to infect, or their immune systems start with proteins that will be better able to adapt to that specific virus, etc.). Certain genotypes and haplotypes confer more (or less) susceptibilty (or protection) to some viruses and this would likely be the case with anything "new" that crossed over from another species. It's all really complicated and of course we're all anthropomorphizing species and viruses and cells and proteins because it makes it easier to understand. Evolution is wild.

On a side note, cheetahs are really lucky. There is an incredibly small amount of genetic diversity in the cheetah population--to the point where I've read that any cheetah can get an organ transplant from any other cheetah. One study gave convincing evidence that the total population at one point was down to something like "no more than 7 individuals"... especially at that extreme bottleneck point, but even up to today, it would seem that they may be more susceptible to an extinction-level cheetah pandemic. Something like that could happen to humans, and things would likely be pretty grim, but our diversity could mean a better shot at existential survival in such a scenario.


Boring_Ad_3065 t1_j584zql wrote

I’d need more context on why you think humans aren’t genetically diverse, or what species you think are more so.

Again, viruses are so simple they aren’t even considered to meet all criteria for being alive. They’re in someways more like molecular machines, and are 100-1000 times smaller than a human cell in a single dimension. Cells are 3D, so ^3 them and it’s a million to a billion times less volume. At that level they (and to an extent bacteria) can swap DNA/RNA accidentally with completely unrelated organisms. Viruses are so “good” at this that all species have “junk DNA” that appears to be the virus inserting part of its DNA into ours and getting replicated (some junk DNA may play an important role we haven’t figured out).

I say this with caution - the vast majority of these swaps are completely fatal, or are worse than the original and don’t survive long. However each infection (one not quickly squashed by the immune system) creates many many billions of viruses in a human (this is likely approximately scalable by body mass).

Humans as a species with a very plentiful population are pretty genetically diverse. We’re actually arguably more diverse over time as civilization broadly allows survival of otherwise less optimal people. For example I’ve got a pretty decent brain on me, but I’ve had very poor eyesight from an early age, and while a healthy adult was sick pretty often as a kid. It’s very possible I’d have died or been destitute if I was born even a few hundred years ago.

And you can actually see this evolutionary selection in earlier humans. Humans everywhere adapted more to their environment the harsher and earlier it was.

  • Skin color: melanin is a defense against UV radiation, which is more intense the closer to the equator you are. Conversely it limits natural vitamin D production, which skin produces from UV exposure.
  • Immunity: the Black Death killed something like 20-30% of Europe’s population. There are certain diseases that Europeans have higher likelihood of resistance than average
  • Persistent Lactase: Europeans had access to more domesticated animals that produced milk, and are the least lactose intolerant group.
  • Altitude: natives to the Andes in SA have far better tolerance to thinner air at 1-2 miles above sea level.
  • Malaria: Africans with great malaria exposure, are at much higher risk of sickle cell anemia, because carrying only 1 copy of the gene produces significant resistance to malaria.

Bottom line is that humans are pretty diverse, and in any case it’s hard to compare genetic diversity of complex species to single celled (or zero celled) organisms.


SethSky t1_j4xduf7 wrote

It's actually pretty complicated, but in general, viruses need certain receptors on cells in order to be able to infect them. These receptors are usually specific to certain types of animals, which is why you can't just give a virus to any animal and expect it to be able to infect them. Additionally, even if a virus is able to infect a certain animal, it may not be able to replicate effectively or cause disease, which is why a lot of cross-species transmissions don't result in an outbreak. In short, it's all about the virus's ability to bind with specific receptors on host cells and replicate effectively.


evogytis t1_j4ykq4f wrote

Virus researcher here. The answers provided here cover most of it - the compatibility between virus and host largely determines the success of a virus jumping into a new species. What hasn't been mentioned much is opportunity. There's probably many viruses like SARS-CoV-2 out there in bats and other animals that haven't emerged in humans (yet) because the circumstances that put them in contact with us haven't happened yet. Once they do we'll be at risk of another pandemic. Likewise, many animals are likely susceptible to some human viruses but rarely get a chance to catch them from us.


supagirl277 t1_j4zcn2c wrote

The way that they infect people are different because animals and humans have different proteins for them to attach to. It has to bind to a specific protein that they’ve adapted to, and if it’s not present, it can’t attach. They have to find a way into the host without getting caught, so they sort of sneak in using mechanisms that the body won’t recognize.


Lexical3 t1_j502ape wrote

more simply: nothing but time and chance. Viruses regularly mutate to become infectious to a different proximal host in a process called Zoonosis. It's a large part of why the current avian flu pandemic is so scary- historically, our worst flus are mutated avian variants!