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A friend told me, during a 3 minute discussion, that viruses that are endemic in host $A$ and make repeated jumps to host $B$ but can't be transmitted between individuals of species $B$, may slowly adapt (through these repeated jumps) to be able to be transmitted between individuals of host $B$ and become epidemic.

I don't know much about epidemiology. I don't understand how a virus population that is endemic to host $A$ may adapt to host $B$ with repeated jumps while the viruses that jump to host $B$ are dead end because they cannot be transmitted further more. Or Are these viruses able to jump back to host $A$ to bring back their newly acquired adaptations to host $B$?

Also, I might misunderstand the meaning of "repeated host jumps transmission". I first thought it meant repeated jumps from a reservoir population in host $A$ to host $B$, but it is also possible that it describes the dynamic of a virus population that is adapted to jump from species to species and they actually gain this ability by keeping jumping and jumping. But then, how could a virus species get adapted to jump from species to species? I'm a bit confused…. Can you give me some hints about this process of cross-species transmission through repeated jumps?

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A friend told me, during a 3 minute discussion, that viruses that are endemic in host A and make repeated jumps to host B but can't be transmitted between individuals of species B, may slowly adapt (through these repeated jumps) to be able to be transmitted between individuals of host B and become epidemic.

This is...mostly true. A good example is avian influenza - there are a number of human infections linked to contact with infected poultry, but the virus is not particularly good at sustaining long chains of human-to-human transmission, likely due to differences in the respiratory tract between humans and birds.

I don't know much about epidemiology. I don't understand how a virus population that is endemic to host A may adapt to host B with repeated jumps while the viruses that jump to host B are dead end because they cannot be transmitted further more. Or Are these viruses able to jump back to host A to bring back their newly acquired adaptations to host B?

What they mean by repeated jumps, or what they should mean, is not a virus hopping back and forth, but a virus going from A to B many times. This is my problem with your friend's statement, it's not so much a "slow adaptation" as, well, random chance.

Imagine that there's a mutation in Virus A, which lives in chickens, that will make it well adapted to live in human beings. It's an extremely rare mutation, so the dominant strain of Virus A is maladapted for human to human transmission. That means most Virus A infections in humans will be the non-productive kind, that will infect a person, but not go on to infect other people. But, with enough rolls of the dice, a human might get the human-favored Virus A, and since there's now intense selective pressure for that strain of the virus within its new human host, it might establish itself and take off.

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A quick answer for now (I'll add details as I find):

The infection of B from A would be zoonosis. This transfer may not have followed a classical route of infection. For example the case of hantavirus pulmonary syndrome (HPS), the zoonosis occurs throuch the rodent (primary host) droppings.

Other example is that of avian influenza- the zoonosis doesn't happen via the respiratory route but by contact with body fluids of infected birds.

Also dead end can be achieved in the case of vector borne diseases especially those of Arboviruses. There is no mechanism of spread other than through a vector.

In other cases the virus may not be produced in high titres in the secondary host to cause infections, but enough to cause mild symptoms.

It is interesting to know how the pathogen can propagate in B but cannot infect: I dont have a definitive answer for this. If a receptor/viral adsorption site is similar between the two hosts but interaction with virus is much stronger in A, then B cannot be infected unless there are loads of virions. However once adsorbed, they can propagate within the cell and go from cell to cell with a relative ease, may be by using a different mechanism. For e.g. , in case of plants the viruses can go through plasmodesmata.

Back jump should be possible unless the pathogen has mutated to fit the new host. In such cases a enzootic infection might happen and A will be dead end. For e.g. in influenza.

This article says that dead-end host (dragonfly) behavior can be manipulated by a certain parasite (Acanthocephalus lucii) to increase transmission probability to original host (perch).

PS: influenza is a very unique case as you might be aware of the genetic drifts/shifts that make it highly adaptable.

Refer this article for some details, It has a section on dead-end transmissions.

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Thanks a lot for your answer. I am not sure I fully understand: Can you tell me what is usually meant by "repeated host jumps". Does it mean repeated jumps from a reservoir population in host $A$ to another host $B$. Or does it mean that virus lineage jumps repeatedly from host to host, from the bat to the dog, from the dog to the pig, from the pig to the human, etc... –  Remi.b Apr 29 at 7:14
    
i dont understand that term either.. didn't get any hit on google.. It might mean the case of influenza where the virus has jumped to and from many hosts. –  WYSIWYG Apr 29 at 7:17

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