system
Given the following post, respond as if you're commenting on Reddit:
human
How can people catch diseases from animals which are not subsequently transmitted from person to person?

I’ve just started reading *The Plague Year: America in the Time of Covid* by Lawrence Wright. In the prologue, he writes: 

> People also get infections from animals, but they are not considered human diseases unless they are shown to be transmitted from one person to another.

My reading (which may well be wrong) is that animal diseases may occasionally be transmitted from animal to human, but such transmission does not necessarily indicate that the disease will then be transmitted from that human to another human.

But I’m struggling to wrap my head around this. Surely the limiting factor in disease transmission is the susceptibility to infection of the *recipient* - and so, once a pathogen has demonstrated that it can cross the species barrier and infect a human, that means humans are susceptible and human-to-human transmission is possible?

If not, what is it that means animal-to-human transmission of that disease is possible but not human-to-human? Why does the species of the infected party matter?
gpt
There’s a lot more to transmission than being able to cause disease. Some examples -

* pathogen may replicate to very high levels in chicken, very low levels in human (where immune response might lead to cytokine storm and death, with almost no viral shed)
* pathogen may replicate in a different area in human and host. E.g. in the gut in chicken (easily shed), deep in the lungs in human (very little shed)
* pathogen may not replicate completely in human, whether because it can’t complete its cycle in a human cell (but can still damage the cell) or because immunity controls it (perhaps with a cytokine storm)
* pathogen may modify its structure in different cells. E.g. glycosylation of viral proteins is host cell-dependent, and is different in different species. 
* pathogen might have to mutate to adapt to human cells, with the mutation leading to reduced fitness. E.g. avoiding human innate immunity is quite different from avian or bat or swine immunity. 

And so on. The most common issue is probably #1, where the pathogen can replicate in the new host but not nearly as well as in the natural host, so that the natural host gives much higher exposure than contact with the new host. This is almost certainly quite common, with many, many instances of animal viruses jumping into a human and not being able to spread, dying out after a single infection - either with an undiagnosed disease, or asymptomatic. Or of course being diagnosed - virology journals are filled with cases of single odd infections of humans that never went anywhere, which is why virologists have been so urgent about warning of upcoming pandemics.  

(We are currently seeing COVID adapting to its new host, for example, so that delta gives much higher exposure and better transmission than did the original bat-adapted strain.).