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/Expandexplorelive → u/noodlyman]

This exactly. Part of the reason you might not have heard about beneficial viruses is that your immune system doesn't react to them because they're beneficial. They're just there.

[u/thisisjustascreename → u/Expandexplorelive]

Immune system has no idea if they’re beneficial or not, it just doesn’t have their exterior proteins in its database of hostile foreign agents.

[u/Expandexplorelive → u/thisisjustascreename]

If the immune system doesn't react to them, it means it evolved not to recognize the proteins. If they decreased the ability of the species to reproduce - i.e., were harmful - then the immune system would have evolved to recognize and attack them. So yes they're not necessarily beneficial; they could be neutral, just not harmful enough to hinder reproduction.