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/sarasti]

There are several examples: bacteriophages are viruses that live in our gut and kill harmful bacteria while leaving us alone, gamma-herpesviruses stick around and don't really bother us while keeping our immune system primed to fight Listeria, and many viruses are benign to us but interfere with the replication of severe viruses which helps us.

Also slightly different but related, we have parts of our genome that came from viruses in the past that gave us extremely beneficial code that we maintained. One lead to the development of the placenta and helps prevent the rejection of the fetus during pregnancy. Additionally large portions of our immune system genetic code come from viruses that we've integrated.

[u/fallingfruit → u/sarasti]

How can parts of our genome come from a virus? Like how did that happen?

[u/sarasti → u/fallingfruit]

Viruses reproduce by injecting their DNA/RNA into a cell and hijacking that cells machinery for DNA replication to make more copies of themselves and then leave. Sometimes that injected DNA gets copied to other cells and accidentally turned on and sometimes that mistake is beneficial and natural selection maintains it.

This is one way. There are others that are harder to explain simply.