What happens if there's a bug in the gene code/splicing, and a self-replicating virus causes a worse problem that it was intended to solve? (this happens all the time in pharmaceuticals already) Or because a virus was manipulated, it can then mutates in a way that in it's natural state, would never mutate? (another unforeseeable result)
Viruses always mutate, and they are far more clever about it than any puny human. But fewer than one in a billion viral species has the slightest interest in infecting you.
