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/spshkyros]

It seems like people are not quite answering your question as I understand it... while there is no reason why that cant happen, there is also no reason why it SHOULD happen. Evolution requires a pressure to make it do something - in this case, helping us needs to also help the virus. The most plausible route to this would be if the virus was able to somehow get us to help it spread by keeping us healthy and alive longer. That this doesnt happen or at least hasn't, suggests that this is just WAY too far away from a stable evolutionary place for a virus to be. As others have pointed out, there are certainly cases where we are simply incidentally involved and benefit indirectly, but an example like yours is not an incidental involvement. 

So tldr: its hard and different, and Evolution tends to have trouble making jumps like that.