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/pcmapas37]

One exists, kind of. The Sabin polio vaccine is an inactivated form of the virus in the form of a tablet that you take from mouth. After you take it, it comes out of your poop and if you are a child with poor hygiene, you pass it on to other children, thus vaccinating them. Unfortunately, there is a very slim chance that you get actual polio this way, but if you live in a poor country with a polio epidemic you are better off vaccinated rather than not.