What will the goals of the disease be? Simply to spread? If so, how patient will it be?
In any case, here are my contributions to making this thing more of a challenge/threatening, in fact this is full on apocalyptic levels of threat if played to the hilt and include everything everybody (including me) are suggesting, and 8th level may not be high enough to play a major role in stopping it... then again, these plans could take two or three years to play out, in which time the PCs would have gained some levels. This could be a "I have not yet begun to fight"/"I have returned, stronger than ever" as the disease smartens up after its initial defeat (having infected a second pool of individuals or animals who made no big move, but were simply a fall-back position). In any case, here it goes:
Once the disease is smart enough (if it is a hive-mind), or always (if it isn't) it should have the option to NOT progress in any given individual, or even allow itself to return to an earlier stage (including "no symptoms"). This would allow it to lay low. Maybe heal check with a very high DC should allow it to be detected even in such cases*. Note that, as far as I know there is no such spell as Detect Disease in D&D.
*Maybe also requiring an alchemy check to be successful as well... you litterally would be talking about a blood-chemistry analysis in such a case, which would very much fit what a high-level alchemist should be able to do in most campaigns.
In any case, you should strongly consider giving it spell resistance against curative spells. Even a flat SR 10 would mean your Joe-Average 5th to 9th level cleric couldn't be SURE in an asymptomatic individual, which, if you go by the NPC populations in the DMG cuts the number of casters who CAN be sure to something like 1/4 (more complicated than that, but that is vaguely in the ball-park).
Actually, now that I think of it, the Black Death had a pneumonic form, and a different form. Giving the disease the choice of which of 4 different forms with radically different symptoms could really mix things up as far as the detective-work goes. Note that the mechanical effects don't NECESSARILY have to be different (Stormwind Fallacy as applied to diseases instead of characters). Dex loss, for example could be trembling, dizziness, or numbness. The disease could make the descision upon infecting an individual, or it could have the option to change its effects (including after a period of laying dormant).
Come to that, the differences in the symptoms of the black death MAY have been based on what vector the disease took to infect the specific individual. Intentional mutations to the vector (perhaps with air-born being a secret weapon it is holding in reserve, or balanced by a lower DC).
Remembering that weither it kills people or the controlled ones get killed by various groups the bodies may still be contagious.
Consider giving it fire resistance, meaning that if a body is burned on an ordinary bonfire it just becomes an inhaled vector, or at least an injested vector via any food, drink (including crops and water sources) it lands on. Kilns or a giant smithy-furnace would still do the job.
Shoot, make it able to crawl through cloth, and perhaps leather... suddenly the best availible gloves and masks DON'T provide any protection for disposing of the dead.
If using a hive-minded, (or at least cooperative to the point of kamakazi attacks) version:
In D&D most people forget about the animals. (Seriously, I have homebrew undead that I designed on the assumption that the most common target for them is going to be a COW, because it is an easy to acquire, large-size creature that nobody thinks twice about someone purchasing for slaughter, and very few people I have asked for feedback on the overall creatures have made the connection... over-the-top Evil necromancers use humanoids, SMART necromancers start with creatures that won't be missed.) Look up how transfer to animals and back to humans gives time for mutations to happen, resulting in last year's flu shot not helping much this year. I think such animal populations are called "reservours". Having it be able to make a decision to conciously hide out in animals to lay low if things go bad is a nasty twist. The other thing is, if the disease's intelligence isn't dependant on the hosts, you can have it infect the livestock in an isolated village while laying low, then start showing mild symptoms in the middle of winter (perhaps having first had a single cow carry a lantern into the largest grain-store in town and burn it down... IE a something that LOOKS like legend of how The Chicago Fire got started). The point being that they would have to pick between eating the meat anyway, and starvation. When the disease shows no symptoms (they might not even infect the humans the first time around) their would be a president for it being "harmless in humanoids".
Bonus points for it only even TRYING to proceed to worse symptoms in individuals who were treated with Heal checks, and going into "remission" after a while if no such attempts are made. It wouldn't want to tip its hand about the SR at such an early stage though.
The priorities would then be to spread (with mild symptoms) beyond the cattle in that area, and start moving into wild-life, and also infect an itinerant druid (or use Rich Burlew's Gleaner, which can be found from one of the sticky articles in the homebrew forum) or three and subtly influence him/her to travel even more widely while giving out "free advice/an interesting story" in every tavern along the way... talking about how the meat is safe to eat and all that.
Then, after it has become an epidemic among the livestock and wild-life, have a new generation of calves (and piglets, and etc) with DIFFERENT symptoms start attacking whatever targets it thinks most important as preparation for its big attack (at least in that area).
The big attack could be via shifting the vector to injury(or in medical terminology saliva-to-blood and/or blood-to-blood). The wolves and farm-dogs kill a few of the herbivores, and the claws and hooves get coated in the blood rendering them into vectors for the disease. Then all the livestock rising up in a single night, as wild animals stream into population centers.
