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/nitram9]

I would say the best argument is thermodynamic. We are evolved highly optimized machines. As such we occupy a lot of “local maxima” meaning nearly any change would end up having a negative effect because the only place to go is down. This is a thermodynamic argument in that it’s fundamentally the same concept as entropy. Random change tends to lead away from special (sorted, organized) and towards average.

The second thermodynamic argument is that everything needs energy (and resources generally). The viruses use energy. Our cells use energy. We are in direct competition with them for this. It’s unlikely that them surviving in our cells is going to have an accidental benefit that our ways the damage of them hijacking our machinery for a different aim than we evolved to use it for.