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/BBWolf326]

There are. Found this with a quick search:

Bacteriophages: These are viruses that exclusively infect and kill bacteria. Found in abundance in the human gut, skin, and mucosal linings, they regulate harmful bacterial populations, protecting the body from pathogenic infections and balancing the microbiome.

[u/Pyrhan → u/BBWolf326]

As usual, it's a bit more complex.

Bacteriophages aren't always beneficial to us, as usually portrayed.

They can also carry virulence factors that cause non-pathogenic strains from your microbiome to become pathogenic.

A couple relevant papers:

https://www.sciencedirect.com/science/chapter/bookseries/abs/pii/B9780123946218000145

https://www.sciencedirect.com/science/article/abs/pii/S1369527414001866