why do viruses move between species sometimes and other times not?
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The virus is designed to interact with certain types of cells of the infected species in order to infect them. It is possible – nay, probable – that the virus gets into a different type of animal and just doesn’t interact with anything in that species because there’s no match for the cell types. No harm done.
Of course, the virus is still a physical thing and could still get passed to a different species again, or back to the same species it originally came from. But this intermediate animal doesn’t care.
It’s actually rare for a virus to be able to interact with multiple different types of animals.
It is in an organism’s best interest to define a niche for itself. That’s what saved the small mammals from being killed with the dinosaurs, they had a niche. The same thing applies to microbes, it’s really great if you have a niche host, that way you can be well suited to that host and increase your chances of survival. Sometimes not having a niche host/ specific host will increase your survival, but it comes at a cost of not being well adapted to your host. Being able to move between species can be advantageous should it happen that one of them dies out, you still have other hosts. I guess it all depends on the needs of the virus and whatever stress it happens to be under.
Viruses need to get inside a cell to use the cell to reproduce, some viruses are flexible on what cells they can enter and which cells they can hijack to reproduce. If they are not adapted to use a different species cells they can’t move between species.
Think of a virus as a criminal walking around with a key, trying to unlock people’s doors and break in. Each person has a different lock on their door, and that makes each person a different species in this metaphor.
If the key just happens to fit a lock, that means the virus can infect that species.
But now, what are the chances that a single key can open up two houses next door to each other? Pretty damn unlikely. What are the chances that a single key can open two different houses on separate sides of the country – probably likely.
So odds are if it’s a single criminal with a single in a single neighborhood, he’s only going to get into a single house. That’s most viruses.
If you have a ton of criminals walking around a single neighborhood, you’re a bit more nervous because it’s not *impossible* that the same key could open two houses. That’s your first cross species jump.
Allowing that happens randomly once, criminals talk. Those two successful original criminals use their profits to make many more new keys and suddenly those keys are in the hands of criminals around the country.
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