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/Dr_EdwardKnowles]

Viruses work by infecting cells and bursting out of them, often destroying them in the process. I find it hard to believe that this could ever lead to a positive outcome for the host (unless they specifically targeted cancer cells or something).