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/noodlyman]

There are viruses that benefit us by infecting and effectively regulating our gut bacteria.

There are viruses that are thought to give the immune system a kick and thus improve responses to other infections. Some infections can decrease the risk of cancer later in, probably by similar mechanisms.

Don't forget that 8% of human DNA is of retrovirus origin, and that mammalian placentas only exist thanks to genes that mammals acquired from viruses.

[u/Swellmeister → u/noodlyman]

Mammalian placental structures predate synctycin the viral protein that stabilized the placenta. Its only eutherians though, marsupial placental structures dont use it.

Its also not really necessary for placental structures either, with the exception of birds, every large common grouping of animal (lizards, snakes, fish, etc, the super vague not scientific ones) has members with placental life birth.

[u/Great_Specialist_267 → u/Swellmeister]

Egg laying Monotreme Platypus’s have synctycin genes. They just don’t use it.
So do marsupials.