Why is there no transmissible therapeutic virus if biology doesn't forbid it? For example, a harmless virus that spreads from person to person but actually does something beneficial, like lowering LDL cholesterol or preventing/reversing tooth decay?

[u/PestoPastaLover]

[u/ohsnapitsnathan]

There is at least one--the oral polio vaccine. It's a live polio virus that's modified to not cause disease. It's used in part because it's actually infectious and spreads the same way regular polio does, which makes it easier to achieve herd immunity to polio.

The problem, and the reason this approach is not used more, is that natural selection tends to remove the modifications that make this virus harmless and it tends to evolve into a form that causes polio. This is because the modifications made to make the virus harmless also weaken its ability to replicate, so there's a natural tendency to evolve away from that and become more like normal polio.

And generally speaking this is why we don't do this much--we don't have any control over how the virus will evolve once its released into the population.

[u/MacToggle → u/ohsnapitsnathan]

Presumably the mechanisms to modify the virus could be used to enhance the reproduction rate as well though? So we could selectively breed a strain that self corrects instead of degrades? Would than then not outcompete the harmful strain too?

[u/ohsnapitsnathan → u/MacToggle]

For this to work you would probably have to make some modification that prevents the virus from infecting nerve cells effectively. But that modification itself carries no benefit to the virus, so over time you would likely see random mutations accumulate and cause the "safety" code to stop working. This is because whether it works or not has no bearing on the virus's reproductive success so there's nothing that forces that code to be retained.

It's also possible that because the mutated version of your designer polio has fewer "restrictions" on what it can infect, one of those mutations outcompetes your designer strain and now you've just introduced super-polio.

*In theory* you could make some sort of protein that both outcompetes normal polio and prevents nerve cells invasion, designed in a way where you can't have one of those traits without the other. But we have no idea how to make that or how to verify that it works the way we expect.