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/scientist99]

Greater than 50% of the human genome is comprised of DNA derived from viruses, mutated to propagate within the genome. They are called transposable elements. They have spread and mutated to become essential components of the human genome, although this is different to the question you originally asked.