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Somewhere on the outskirts of Tang China, in a tribe of nomads that would have otherwise gone completely forgotten by history. One member of this tribe will rise to heroic proportions and become the greatest conquerer in the history of mankind. He came from the humblest of origins, a true underdog story.

Deep in a horses colon a king is born. Several generations of humans eating contaminated horse meat have led to a nasty strain of equine influenza with an incubation period of 8-15 days, and the ability to transfer almost harmlessly through humans. I know next to nothing about protein classifications and that kind of thing, so until I come up with something better this virus will go by HNR1 or 'Henry', for short.
Humans affected by Henry have a 99% survival rate except in sick, elderly, and infant populations. Homo sapiens can pass the virus onto their horses for nearly a two month period, even after all symptoms have resided. Common symptoms are migraines, respiratory irritation, fatigue, and diarrhea. The latter contributing to rapid transmissions from tainted water sources.

(Let me be clear I have no personal grudge against our hoofed companions!)

Amongst infected horses the situation is far more grim. I'm basing the effects on equines on the information I got from this website.

http://www.equinetrader.co.nz/articles/controlling-equine-influenza-jpn-/#.UZlRYivwJMY

[Thursday, April 15, 2010
A recent study confirmed that steps taken to manage an equine influenza outbreak in Japan were effective, working quickly to control the outbreak. On Aug. 15, 2007, 19 racehorses stabled at four race tracks came down with fever. Within a month, equine influenza spread swiftly to other tracks infecting 529 horses." "Outbreaks can happen even among fully vaccinated horse populations, although this does not mean vaccination did not work at all," Nishiura said. "Vaccination can reduce the risks of symptomatic disease and severe manifestations." In addition, the 2007 outbreak would have been much larger if the horses had not been vaccinated, he maintained. In the 1971 outbreak, which affected largely unvaccinated horses, the proportion of febrile horses ranged from 81.9-99.4% of racehorses, far more than 12.8% in 2007.]
Holy shit! up to 99%!? The people of 900 CE will certainly not have access to any vaccines. Human beings will do most of the hard work for the viruses anyway. Now I'm not a complete monster and don't want to wipe horses off the face of the earth, they did nothing to deserve that. I'll nerf that number down to 95. Wild populations will do significantly better.
Horses afflicted by Henry will experience high fever, diarrhea, extreme fatigue and eventually fluid buildup in the lungs. Most horses will die of dehydration or malnutrition, pneumonia will finish off the rest. I expect there will be massive exterminations in a vain attempt to stem infections. Any remaining domesticated horses will probably be eaten by their starving owners in the ensuing collapse.
The Human response will almost certainly make things much much worse. I can't think of a better place for Henry to flourish than a stable. It's going to take a very long time for anyone to realize we are the ones making the horses sick. By then it will be too late, just think of every horse in Baghdad simultaneously becoming too fatigued to move and in most cases eventually dropping dead.

I spent all morning thinking about the implications of such a plague. I'll leave you guys to come up with your own ideas before I share mine.
A biological POD like this would only require a few misplaced molecules and a malnourished human host who neglected to wash his or her hands before eating horsemeat. Far less intrusive than any changes that would have to be made to allow...a confederate victory for example.
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