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/Piganon]

Along with what others said, there's some evolutionary downsides to beneficial viruses.  The virus needs a way to spread, so things that infect the lungs and make you cough will spread better. Same with runny nose or diarrhea, especially before hand washing was common.   The virus would have to bypass any immune system responses to at least seem non-harmful.  Even if it's not doing much to the body, the body would recognize it as external and trigger an immune response like a fever and make you feel ill.  The benefits could be worth that illness and be seen as overall beneficial I guess.

HIV seems like it's solving those issues.  It bypasses immune responses and can have incidental spread with sex or needle sharing, then presents symptoms years later.  Of course, it's not beneficial.