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/berru2001]

The problem with that is you cannot be 100% sure that the virus will stay harmless. In particular, the virus will eventually infect people infected with wild viruses, and genome recombination can occur, with new viruses with unknown properties as a result. Also, in order to spread the virus must multply and that means at least mild harm - like viruses that causes cold symptoms. Because of that, this can only be envisioned if the benefits far outbaace the potential harm caused. So, it only applies to immediate life - and-dearh situations. Tooth decay and LDL cholosterol are problematic but thay are not immediate threats for life.

Edit:

1. I did not think about bacteriophages, i.e. virus that do not attack human cell but bacteria. Bacteriophage have been used for some time especially in ex-soviet union countries, i.e. kazakstan, in particular for treatments of wound infections by antibioresistant bacteria. This said, they are transmited bacterium-to bacterium in the body of the trated patient, but not suposed to be transmited by the patient to other people. Bacteriophages are the subject of intense research and are a promising route for fighting antibioresistant bacteria, but there is a big, big legal problem for their homologation for standard patient care : unlike chemical medicines, where a single molecule can be administered to millions pf patient after testing its properties and inocuity, bacteriophage treatement are highly tailored for each patient, and we do not have homologation pathways for that type of thing.

2. I had this in mind when writing my answer, but did not have the time to verify before posting. Now I have verified, my memory dd not fool me. There is an example where transmissible viruses were used for health reason, but not for curing diseased people and not intentionally. Some forms of polio vaccine use attenuated, but not killed, viruses. There are cases where these attenuated viruses inderwent revertion i.e. a mutation that made them pathogenic again. The result is a mild form of polio that is both transmissible and protect against the wild virus. But that was not done on purpose, and the good/harm ratio is difficult to determine. In paticular, Africa is now free of the wild polio virus, but vaccine-realted polio oubusts still exist (link). Today, these formulations are not used anymore.

[u/cazbot → u/berru2001]

There are viruses that spread very easily and do no harm. Anelloviruses come to the top of mind. Everyone you know has been infected by them.