The rats, which lived among humans in cities, died off. But the fleas still lived, as did the plague bacterium. Once the disease broke out in human society, it flourished with predictably deadly consequences. From bacterium to flea to rat to livestock to human to human. This is a fairly typical vector-pathogen relationship.
Diseases become so deadly when they slip outside their normal ecological niche.
Living organisms, particularly lower forms like bacteria and viruses, take full advantage of any evolutionary niches created for them. DNA is a self-replicating chemical that, once set in motion, will continue to replicate and diversify. The state of the Earth at this time is proof of the versatility, pugnacity, and success of this molecule. Of course not. But I strongly question whether it s even the best method to do anything except enrich biotech firms and their stockholders.