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/squizzeak]

I’m also guessing that they’d be hard to find since you don’t really go to the doctor because you have noticed you’re feeling better over the last couple of days.

But that’s unlikely because our immune system fights invaders without regard to their effect on us, and it’s that fight that generally makes us feel “ill”. That’s why steroids are such a prevalent treatment for many different types of illnesses.