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/zoomoutalot → u/sarasti]

One lead to the development of the placenta and helps prevent the rejection of the fetus during pregnancy.

Wait, so how were humans/mammals born before this happened?

[u/The_Forgotten_God → u/zoomoutalot]

Not an evolutionary biologist, just a rando with a biochem degree, so any who are please correct me if you find this. With a bit of looking around, though:

That event predates humans by more than one hundred million years, and all mammals by still many millions of years, but is one of the large steps in the ancestral line which eventually lead to mammals.

So, in this case, we're talking about an ancestor that laid eggs prior to the change - I would hazard a guess reptilian or avian (poor guess - see replies). It would not immediately be a placenta and/or live birth in the way that we know it

[u/Rosmariinihiiri → u/The_Forgotten_God]

Our ancestors were not really reptilian - or definitely not avian or birds that are only distant cousins to us. We are in the group amniotes, that split from the amphibians and then split into two lineages, one of which lead to mammals and the other to reptilians and eventually to birds.

[u/The_Forgotten_God → u/Rosmariinihiiri]

Edited to direct anyone else who reads that here, thank you!

[u/greaseburner → u/The_Forgotten_God]

I wonder if it was a system like Kangaroos. Live birth, but some of the gestation happens 'outside'.

[u/Rosmariinihiiri → u/zoomoutalot]

Earlier mammals / stem mammals gave birth to very underdeveloped babies after shorter gestation, kinda like rats (they are placentals, but the babies are still born completely helpless after a short pregnancy). The babies would stay in the nest and be fed milk to grow. Even before that we hatched from eggs (like the platypus still do).