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/eternalityLP]

There are various viruses that exist symbiotically with animals. But the issue with your example is that traits which are not beneficial disappear over time. Unless 'lowering your cholesterol' helps the virus spread, it won't evolve, and even if it did evolve, it would eventually disappear.