The Disease is viral in origin ; the first symptom of infection is extremely high fever, clinically reminiscent of viral meningitis. If fever is not symptomatically relieved, hyperpyretic brain damage occurs almost without exception, and progresses to coma and eventual death for about one case in three. Fortunately both acetil-salicylic acid and 4-hydroxyphenyl-acetamide effectually control the fever even in small doses ; anecdotally ice packs and even damp cloth are also effective. As the sufferer seeks coolness instinctively, and effectual medicine is widely available the world over, actual deaths from The Disease are indeed rare.
For reasons poorly understood, but shown by Williamson-Shu-Klein to have a probable link to interaction between the viral decapsidification process and specific proteins encoded for by virtually all Y chromosomes, male humans of most lineages (and certainly of all surviving lineages) can not develop the disease. Even if infected with significant viral load, the virus is broken down metabolically and entirely removed from the body over six to eighteen months.
In females, however, the effects of infection are quite profound. There's significant apoptosis of neurons located within the left prefrontal cortex, the left temporal cortex and the left parietal cortex as well as significant reorganisation of white matter throughout the brain. These together result in a drastically modified outlook of the sufferer. No studies of Diseased patients to date have identified any one with an IQ over 90, and without exception the high 80s were measured in recent infections that fell as the Disease took its course towards the ~70 average generally seen in patients.
The poorly understood anatomic changes of the brain, supported by significant changes in the hormonal balance and function of the sufferer that are in turn poorly understood (outside of anecdotal if abundant reports that hormone-based birth control does not work in the Diseased - a large clinical study is underway to confirm this fact, meanwhile universally accepted by clinicians everywhere) drive specific behavioural changes that have been amply documented.
Perhaps most shockingly, the Diseased loses the ability to recognize faces, including her own. She will engage her own image reflected in a mirror in conversation, and will attempt to nurse and parent on an animalistic, basic level any young in the immediate environment (in many documented cases, without much distinction made for species boundries - so much so that stories of the lady of the house found breastfeeding her infant and the family dog, or of how "she's happy with the TV, but happier with a mirror" are now a mainstay of "comedy" acts as practiced for TV).
The erogenous zones become extremely sensitive, and take up most of the sufferer's attention. She generally can not bear clothes, but certainly a belt, a choker or a cape are more tolerable than a bra, or any sort of pants. There are no known cases of a sufferer managing a whole day in an outfit that'd be properly called decent. The causation appears to be more complex than direct innervation, because smooth fabrics such as silk and even radical measures (novocaine) relieve intolerance only temporarily. There's also a constant, all-consuming desire to be looked at by others, which is why sufferers will tend to seek out groups to present themselves to, as much as their limited executive abilities allow (anecdotally, wallpaper made of images of people looking at the camera work well to alleviate this behaviour, which has brought about a major resurgence in royalty receipts for Jason Priestley and similar bad 1980s mass media productions).
Finally, laryngeal secretions become more abundant and, uncommon in humans, travel towards the glossa, making saliva much denser, and more lubricating. The normal mucus secretion in the colon - especially the rectum - becomes much more abundant, exceeding the lubrication potential of the vagina, marked as that is in Diseased patients.
In females infected transplacentarily there are further changes. Ironically, the intellectual impairment is not as severe, with low to mid 80s IQs being almost universally the norm, yet the peculiar behaviours noted before are even more marked, with numerous variants that have yet been poorly studied. The femur and especially the tibia and fibula grow longer than is the norm, with the femur representing 29 to 30% of the full body length rather than the usual 26.7%, and the tibia slightly longer than the femur, resulting in a specific gait and appearance of the (very tall - 6 foot being the average measured height of adult pre partum infections) sufferer.
For all its drawbacks, The Disease is probably involved in a whole range of immunities acquired by the sufferer, as the incidence of ischemic disease, obesity, several notable types of cancer as well as degenerative neural diseases is half (first) to below a tenth (last) as compared to "healthy" control groups.
Anatomically, the virus is rather large, with an extremely complex 5,3 (k5k6akdk5aD) capsid. Its genome is very complex, and not entirely stable. Genome sequencing has produced some results, the most interesting of which being the high proportion of genetic material not previously encountered in any known species, and the extremely high proportion of actually expressed and expressible proteins encoded for - upwards of 70% of all genetic information encapsulated by the virus is estimated to be actually used!
There are no known cures for The Disease so far. Infection does not yield immunity, the common antiviral agents do not harm the virus even in vitro - in fact it is an extremely stable compound that could, in principle, resist complete dessication under a vacuum for millions of years, which has encouraged speculation as to its extraplanetary origin. Efforts to contain and perhaps one day cure the infection are of course underway, at least in the more civilised segments of society, even as new pseudoreligious cults and other covens of abnormality are fighting for acceptance of The Disease as "divine" or otherwise "an improvement".