Necrotizing Fasciitis is a serious disease, but the bubonic plague is called the Black Plague because it is so virulent that it almost wiped out the population of Europe in medieval times. The bubonic plague has had a much greater impact on human history, and therefore is a much worse disease overall.
Necrotizing fasciitis is harder to cure than the bubonic plague. Plus, humans conquered the plague 500 years ago after it wiped out a third of Europe. With modern disease control, necrotizing fasciitis is far worse because it is more difficult to control. Contemporary horror stories show regular people aren't ready for the disease as it is often not detected until it is too late and limbs are amputated to save a life. Either disease is rough, but hardly anyone dies from both conditions with modern medical care.