The most rare and deadly form was the scepticemic plauge, with almost 100% mortality rate. This literally caused the blood to coagulate in the veins, causing the skin to turn black (thus the name, the Black Death). This is most similar in symptoms to a hemmoragic fever, like Ebola.
As the plauge progressed across Europe, it changed to a pneumonic plauge that attacked the lungs. Victims coughed up bloody mucus at first, then as the lung tissues broke down, the liquid became clear red fluid, flowing freely. Death quickly followed. The pneumonic plauge had a 90-95% mortality rate. Also, this type of plauge was air transmissable, and thus extremely infectious. Was this a rapid mutation of the bacterium/virus, or was it another bacterium/virus infection taking advantage of the victims of the original plauge and their overwhelmed immune systems? Thus, in its later stages, it could well have been a multiple organism plauge.
The plauge killed between one-quarter and one-half of the entire population of Europe. If it was in fact bacterial in nature, as originally believed, then a new outbreak could be effectively treated with antibiotics, unless a highly resistant form emerges or is genetically engineered. On the other hand, if it was viral in nature, it would be extremely difficult to treat. There are breakthroughs occuring today in anti-viral drugs and therapies, but right now vaccines are the only effective and feasible large-scale tool to control a viral plauge. Developing a vaccine takes time, and it takes even long to produce in large quantities. Once you've started to manifest serious symptoms, it9;s too late for the vaccine to help you. A new and unknown highly virulent and infectious viral plauge would be devastating, possibly as much so as the Black Death of 14th Century Europe. This is especially true of such an organism purposely introduced as a bioweapon, as those wielding such a weapon would certainly release it at multiple infection sites in places where it would be certain to be spread quickly, resulting in widespread infection before symptoms began to manifest in the first victims. This would make quarantene efforts very difficult and far less effective, as the plauge would be pandemic before we even knew it was present in the population.
One thing that such a modern plauge would likely repeat from the Black Death plauge is the total breakdown of the social order. Everyone would be afraid of everyone, and thus avoid all human contact outside of their own small intimate social group, family and maybe a few friend and neighbors. Of course, food shipments to cities would screech to a halt. The various governments would step in, but their effectiveness would be limited at best. Fear, anarchy, infrastructure breakdown, crime, emotional breakdown, even conventional or nuclear retaliation against suspected perpetrating nations, all would cause as much chaos as the disease itself. With modern citizens being so dependant on society and incapable of taking care of themselves, it would get very ugly very quickly.