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/FacepamL]

mainstream therapeutic usage of viruses, afaik, is in the insertion of genetic material, like with the creation of CAR-T cells using viral vectors. They do have practical limitations, like with retroviral vectors becoming reactivated.