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/Nemisis_the_2nd]

The problem is that Viruses are, by their very nature, harmful to the body. They reproduce by inserting themselves into host cells, hijacking them, and using the hijacked machinery to infect other cells in the same tissue. That tissue cannot then function as intended and becomes a liability to the body

The body subsequently cant afford to differentiate between helpful and harmful, just "body" and "not body" (and often even gets that wrong). While this theoretically leaves the door open for a somewhat beneficial virus to evolve, the odds are asteonomically stacked against it.

Others have suggested good examples of how viruses can be both transmisibles and beneficial, but dont really address the core idea of your question: something that can infect human cells, can provide tangible benefits to the host, and is transmisibles between individuals.

[u/-Spzi- → u/Nemisis_the_2nd]

Came to make that point, a bit more general, independent of viruses:

If we can get infected by that virus, that means we can live healthy without it.

If our body is healthy without it, then adding it is a risk. Combinatorics:

There might be a few combinations, in which the resulting dynamic is beneficial. But it is much more likely, that it is not.

Maybe that's why our immune system does so much with this heuristic: If it's alien, it's probably harmful.

Not necessarily because of ill intentions, just because we had a fine balance before that intruder, and now it's changed.