Yea and it's not the middle ages either and there are no strains of plague that are anti-biotic resistant. The only Bacteria that are scary are anti-biotic resistant ones, all the rest can be cured with a dose of anti-biotic. That's why people with the real knowledge don't research bio-weapons from bacteria, they use viruses that have no effective treatment option.
OTOH, it's far easier to cultivate bacteria than viruses. For example, Yersinia pestis, the bacteria that causes bubonic plague can be grown in a modified agar gel with no need for host cells of any kind. And it's pretty easy to breed in resistance to anti-biotics by exposing the bacteria over many generations to all the anti-biotics in use at doses where a small part of the colony survives.
Whether that can be done over a short enough time that interests an organization like ISIS, is unknown to me. But it wouldn't take much effort IMHO to make a bubonic plague variant that is at least highly resistant to anti-biotics. Making it also highly infectious and lethal is another problem. That might require substantially more testing and breeding of the bacteria in host animals like rats or mice or something closer to us, like monkeys or people themselves.