Yiffers.

Yiffers go by many names, furries, fur faggers, degenerates, the colorful horde, most people go with yiffers. No faction is more reviled, hated, feared, and disgusted by most races in both the old and new worlds. No one knows if it was dark magic, a twisted form of science or a combination of the two. 

Yiffer creation / spread.

The way the yiffers spread so far. One method is ripping off fur and attaching it to a person forcibly being a very direct method not all yiffers have this ability or use it all that often. Their preferred method spread is an unwilling yiffing (rape). Since it is similar to aid's they have us been reports of projectiles coated in there fluids doesn't seem to have any effect on bullets things like blades or arrows are something they commonly use for this purpose.

Yiffers if given enough time and numbers will form packs (or orgies depending how you look at it). If these packs not dealt with quickly those overwhelming number of bodies can very quickly overwhelm population centers if defensive measures aren't put in place.

Stages
Stage 1, have just been infected via blood transfer by blade or other means more by an unwilling yiffing.  No external changes at the this stage. Research is currently ongoing to find methods to potentially reverse the process at this stage depending on how much was transferred to the victim.

Stage 2, smaller external changes start to occur. Hair changes to fur, said fur starts to grow on more places in random colors. Also felt is joint pains from bone and muscle restructuring. The stage if enough effort is put into it can be hidden from a distance.

Stage 3, at this stage there's no hiding what the victim has become. Physical features have changed into more animalistic shapes, but the various patches of fur growing sporadically all over the victim. His humanity gone at this point but not a complete furry yet.

Stage 4, fully covered in fur at this stage, from a distance looks like an amateur taxidermy job gone wrong or genuinely look like a furry. With an Aura of degeneracy that attracts stage 1-3 victims to form a pack and may become the alpha male competing with other stage 4's until a stage five shows up. For most victims at this stage this is their final form, 

Stage 5 research is currently ongoing at this time and has yet to be concluded at this time.

 
Origination/ground zero/known history.

 Most people can agree that the first encounters began in the old world, a what seemed to be the last hurrah for a irrelevant ideology, that being the furry fandom. In a city that let everyone in regardless of race or danger to the populace. That City being Seattle, Washington which in itself was becoming less relevant by the day even with its high ideals. This particular event however put Seattle back in the map that being rainfurest 2 being held at what used to be Paris Hilton next to the Sea-Tac international airport. Which along with hosting rainforest also had its normal guests and a few other smaller conventions unfortunately inside. Most really can't say what happened on the inside is most details vary in description for the few that made it out. One thing is certain the unfortunate denizens of Seattle were plagued by these new abominations. The yiffers quickly spread throughout Seattle due to years of politically correct dumbing down of the authorities and the rampant homeless problem. May have doomed Seattle from the start further cementing it has a black mark on maps as  ground zero and a no man's land. For a while the remanents of the US government and other interested parties interested in stopping the spread. Were able to block off half the state and surrounding waterways. Until the convention center disappeared with three quarters of the current yiffer population inside. At first surrounding forces so this is a victory and debated going on an offensive to rid of them once and for all. That was until reports came back of a deformed convention center popping up in random towns, villages and cities on both the old and new world sporadically and without warning overwhelming any unprepared forces and dragging whatever citizens they could back to their convention center. It usually appears quickly without warning, sometimes it disappears just as quickly other times it has been recorded to have stuck around for as long as half a year in some places if not discovered long enough. This has resulted in yiffers setting permanent footholds in areas other than Seattle. They have not succeeded in doing this in the old world as much however they seem to have had more success doing it in the newer fantasy world which for the most part is unprepared for for these unfamiliar creatures. no one knows how big the convention center interior currently is but it has expanded drastically from the original configuration and size.