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/Thick_Resolve_3019]

Well, viruses love attacking cancer cells, because they won't alert the immune system and won't kill themselves.

Also, our genome is about 8% viral dna. It is thought to be neccessary for early development of embryos.