The ideas behind the common concept of the "bioweapon" as a mass and indiscriminate tool for death are rooted firmly in our modern day culture with sanitation and public hygiene. A bioweapon is only frightening because it's something that circumvents all the work that medicine and science have done to eliminate the plagues of Old that did exactly what bioweapons of Now do. Many, many times over thousands of years. Especially in a genre fantasy setting, it would be hard to make a bioweapon stand out.
As is clearly evident, plagues never killed absolutely everyone. In a historical fantasy setting a bioweapon with our definition would be no different from a plague suffered in some backwater in the Middle Ages. I'm doubtful of Made-in-America™ Super-AIDS's ability to wipe out anyone outside of a ten-mile radius of the local bath house. Ultimately, super-ultra-mega-AIDS would just be another plague to your regular medieval peasant, if at all. It would really only pose a threat in some faggy Goodreads assrape-with-magic gay adventure.
When disease overtook a place the people saw it related to the sins and misfortune of that city or country. The cloud of sickness and death, while tied to the righteous judgement of God, could be restrained by spiritual means, or warded-off by apotropaic magical practices. That's why medicine and care of the sick was tied to the Church in Europe, and with other religious groups in different regions. For many pagan traditions their shamans and wisemen were also healers, because your flesh is afflicted with pain and suffering from an invisible cause. From a materialist perspective, sometimes they administered the right poultice, or had the sick ingest the right herb. But for them their practice in every way was ritualistic, meant to generate a metaphysical effect to relieve both physical and spiritual suffering.
The root word of "pharmacy" is the Greek word"pharmakon." It's meaning is tied to medicine, but also to poison and sorcery. The old healing rituals of old became renaissance hermetic beak-mask medicine, which became modern materialist medicine. The knowledge is no longer hidden and passed down in secret, but still only the learned and vetted are given license to practice.
All of this is why purposeful plagues and diseases are often tied to curses and other evil magical methods in fantasy just as they would have been in the past. It also opens a narrative up for the cure to also be magical, often joined to a specific object or substance to be sought after to generate conflict.
In any case, my fantasy bioweapon would be a disease where if you get assraped by someone infected, you catch it. Nothing happens at first, but on every full moon afterwards the afflicted become ravenous gay rapists that prowl the streets at night looking for fresh rapeable cheeks. A major plotpoint in the story to feature this disease would be that the afflicted men are separated into two groups: those that band together every rape night secluded in fantasy gay clubs, and those that enjoy giving the gift to men unlucky enough to be caught in the light of the full moon. In the end both groups are the real heroes in their struggle against the normal guys who don't want to be raped and turned gay. That's the real crime.
