why do viruses move between species sometimes and other times not?
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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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