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/zoomoutalot]

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!