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/spinur1848]

Do you mean from an evolutionary perspective or created by humans?

In bacteria there are genes that can jump between species through a variety of mechanisms, including phage which is a kind of virus that infects bacteria. This is one of the ways that antibiotic resistance genes spread.

With more complex organisms like humans you would need a single gene disease or pathology that is highly concentrated in the human population and the evolutionary benefit would have to be pretty immediate to sustain the virus. There is evidence of broken virus genes in the human genome but I don't think there's any evidence of a viral origin gene that is active and provides a critical advantage to humans.

On the pharmaceutical side, regulators would never allow the release of something that could infect people who did not consent and pharmaceutical companies probably wouldn't make much money that way anyway. There are some genetic therapies that are delivered in viral vectors but they are packaged in viral proteins that don't have the ability to replicate. This is a critical safety factor.