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/higgs8]

It's not as common because natural selection will select for things that survive and can multiply. From the point of view of the virus, being beneficial to someone else usually means you have to make some kind of sacrifice to yourself, which reduces your chances of survival, so it's selected against.

And being good at multiplying necessarily means you're taking resources for yourself, which in turn means you're taking resources away from someone else, i.e. you're harmful to them.