Magic has influenced the growth of every area of science and technology, including biological warfare. Over the years, mad wizards (or disciples of Tallakath, Klepnos, or some other daedra) have succeeded in bringing to life magical creatures on the micro-scale. These creatures are a little like minor faeries, except that they're the size of a bacterium and thus are invisible to the naked eye. (Many of them are naturally invisible, as well, and won't even appear in a microscope -- they're visible only by their magical aura, and then only if you know what to look for.) Like mundane bacteria, they multiply in a host's body (usually feeding off the host's internal mana reserves) and then pass out into the world by various means, depending on the type of bug being considered. These magical pathogens have been given the whimsical name nanopixies, often abbreviated NPs or nips.
In the last twenty years doctors and medical researchers have found more constructive uses for NPs, "re-training" them for use in gene therapy, cancer treatment, and high-precision body modification. The military has experimented with them as weapons, but results have often been less than promising -- if you make a strain of NPs that eats tank treads, for instance, you might find your own armored columns breaking down when the wind shifts. One other area where NPs have found great success is in cleaning up industrial toxins; be it an oil spill or radioactive waste, a properly-trained pack of nanopixies can leave the place spotless within hours, days, or weeks (depending on the size of the mess).
Some examples of magical diseases:
Vampirism was the first magical disease, and is still the best-known. Its history and effects are described in the article on vampires. Unlike most of the later magical diseases, the vampirism nanopixies are fragile and susceptible to life-aspected mana; they can only take hold in a body that is very recently dead or on the verge of death. Vampirism is a blood-borne pathogen, and can only be effectively transmitted by exposing the dead or dying person to the infected blood of a vampire (usually by introducing it through the person's mouth, though other avenues of infection have been documented).
Accelerated Gynomorphic Syndrome ("Feminax")
Created in the 1950s by Jaelin Kahl, a radical feminist wizard who
lived in northern Sathmore, Feminax was one of the byproducts of the "culture
war" that took place during that time. The Empire had always been a land
of equal rights for women, of course -- not only did it have the Curse
of Metamor as part of its heritage, but the Majestrix herself was female.
Still, fluctuations in culture are commonplace, and the 1930s and '40s had been
a time of increasing emphasis on a woman's traditional role in the home. Careers
in the busy, industrialized workplace of the "modern world" were seen
as unladylike and generally frowned upon. The 1950s saw a backlash against this,
as a generation of newly "liberated" women rose up to reclaim their
rights to choose to live in the "working" world. Some of them went
over the edge.
Jaelin Kahl was one of those who crossed the line from asserting equality to
proclaiming superiority. Not much is known about her background, but it is thought
that she came through great personal trials and challenges to become a high-ranking
wizard in a time when the guilds were dominated by men. It is known, from her
writings, that she hated men, considering them inferior even to her own cat.
Whatever her reasons, she unleashed Feminax on the Empire and then disappeared,
either by polymorphing herself into a new identity or fleeing to some dark corner
of the globe. She was never found or heard from again, but her legacy remains.
Feminax is a magical microbe that is spread by intimate contact -- exchange
of bodily fluids, sharing of needles, etc. Its initial effect is a modest increase
in the victim's libido, which often goes unnoticed. This effect remains indefinitely
until it is removed by dispel magic. Within 6 hours of infection, the victim
is contagious. After an incubation time of 15-30 days, if the victim is male,
he is transformed into a beautiful, fertile young woman. This transformation
is permanent, and resists any attempt at reversal -- it appears to be a derivative
of the androgyne variant of Metamor's Curse,
but it will affect even people who have been Cursed already. Polymorph spells
cast on the victim will only work for a maximum of 28 days -- when the time
comes for the victim's period, he changes back into a woman. Dispel magic renders
the victim non-contagious but does not reverse the transformation. Women infected
by Feminax do not undergo any transformation, but remain carriers of the disease
until it is removed by magical dispelling.
Because its effects on women are so subtle, many women persist as carriers
for years without being identified. Because the disease leads to an increase
in sex drive, a man who has been infected can have sex with many partners --
who can, in turn, have sex with many other partners, and so on -- before the
first victim is transformed. There is no way to know which of the man's partners
was the original carrier. Feminax outbreaks are always followed by careful investigations
by Imperial health inspectors and Red Spiral
workers, but inevitably some carriers fall through the cracks (and some victims
are too embarrassed to come forward). Thus, there is always an underlying "source
population" hiding somewhere, waiting to strike again.
To this day, nobody is quite sure why Kahl created Feminax. Some say that she was a lesbian who wanted to increase her supply of potential partners. Others say that she wanted to teach a lesson to men who slept around on their wives -- it is said that the first people she infected with the virus were prostitutes at a number of "houses of ill repute" throughout the Empire. Still others say that, in her own twisted vision of the world, she was performing a humanitarian service -- allowing wretched, inferior males the opportunity to ascend to a higher state of being. Whatever the truth may be, one thing is certain: the specter of Feminax makes a lot of men think twice before calling up that escort service or jumping in the sack on the first date.
Female Hypersexual Syndrome ("Bimbosoma")
This chilling disease was developed in the 1970s by the wizard Gamlin Durnst
as a revenge against womankind for the creation of Feminax. It has many of the
same properties as Feminax, except that men are the carriers. Women who
are infected with Bimbosoma are transformed, after 15-30 days of incubation,
into young, beautiful nymphomaniacs. They don't exactly lose any of their intellect,
but their minds are so overtaken by the need to mate that all other impulses
are secondary. Their primary driving goal is to pleasure men (or, for that matter,
any willing sentient partner) as much as they possibly can. Additional side
effects include an immunity to nearly all known diseases, and bodies that do
not age at all until their last few years of life (much like the pedomorph
variant of the Curse of Metamor).
Fortunately, unlike Feminax, Bimbosoma overtakes its victims gradually. The
mental changes begin to slowly take effect throughout the incubation period.
While the woman herself rarely notices these changes, her friends and relatives
sometimes do; if the disease is identified at this stage, it can be cured by
magical dispelling. Once the incubation period is over, though, and the physical
transformation occurs, there is no cure. Some wizards are experimenting with
amulets that diminish the mind-altering effects of the spell, but no "perfect
solution" has yet been found. Part of the problem is that the women who
have been transformed by Bimbosoma don't want to be cured, and since
they retain their mental faculties (they can still drive a car or skimmer, prepare
meals, fill out a tax return, and perform at least some types of jobs without
any difficulty) they cannot be legally committed to a mental institution without
their consent. And so they go about their lives, hopping in the sack whenever
they can, oblivious to the grief of their friends and families who have lost
the woman they love.
The really ironic part about this whole story is that Gamlin Durnst -- who was eventually caught and sentenced to life imprisonment for unleashing his creation on the world -- was infected with Feminax by a guard who slipped it into his food. After his transformation, he was transferred to the women's ward -- where he was infected with Bimbosoma by one of his fellow inmates. The guards have, in the years since, often described in gleeful terms how the former man went into an emotional breakdown as she realized what was happening to her. After 27 days, however, she really didn't seem to mind anymore -- and once the Feminax was purged from her system, she became very popular throughout the prison, with staff and inmates alike.
Rumored to be created by Klepnos, this is a rather minor illness that typically affects people early in childhood. It begins with rashes and small bumps that soon spread all over the skin, accompanied by a general feeling of fatigue and the occasional upset stomach. After about three days, the bumps break open and sprout white, fluffy feathers all over the child's body. The secondary feelings of illness disappear a few days later. Many children develop a sudden hunger for corn after their appetite returns. The feathers persist for two or three months, gradually falling out, and leave behind perfectly healthy skin.
Those who have had chicken pox once cannot get it again, although "flare-ups" are a common occurrence later in life (from about the late 40s on). These flare-ups tend to be more severe than the original illness, sometimes turning the person entirely into a chicken, but the effects are short-lived and disappear after a few days. There is no cure, but people generally consider it more of nuisance than a real hardship.
There are also "diseases" that are not caused by any actual magical "bug", but by contact with magical beings that leaves a nasty lasting impression. This type of illness is more like a poisoning than a true disease. The "devil chills", caused by contact with certain kinds of daedra, are the classic example.