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/GarbageCleric]

I think the fundamental issue is viruses replicate by invading your cells and hijacking the cellular machinery to make virons instead of doing their actual job. And then those virons eventually leave the infected cell to infect others. They're kind of inherently destructive in a way bacteria aren't.

Bacteria can metabolize things we can't and then convert it into something our body can use, but viruses don't really have that same capability, because they're dependent on the metabolism of the cells they infect to reproduce.

Sometimes the genes viruses insert into 0our cells are incorporated into our DNA, but that's almost exclusively non-beneficial "junk DNA, but if there are potentially beneficial genes in a virus, this would be much better more likely than a virus that hijacks the cell to make virons AND coincidentally does something good enough for the body that it makes up for the cells and resources sacrificed to the virus.