Changeplague

A disease that causes Shifters to pass on their transformation abilities to non shifters they bite.

Origins

When Shifters were first created, their bites carried no lasting effects beyond the physical damage. This would change during the Third Era, when the Abombnelyths used Shifter tribes as pawns in their wars to destroy Nagusacktem. Shifters were impressive physical specimens and powerful warriors, but they were not very numerous, and their numbers increased slowly. Combined with their low frequency of wizarding ability, they would struggle to defeat the elves of the Arcane City. 
While the Abombnelyth Sinastric worked to bend the local shifters to their will, its sibling Mulephicent labored to increase their numbers. Rather than boast the Shifter birthrate or maturation speed, it created the contagion that would alter Shifters forever. Any Shifter infected with this Changeplague would pass a version of his shapechanging abilities to any victims of his bite. The main effect of this would be the forced transformation during the full moon, though with training and practice a victim could voluntarily change at anytime. Unfortunately, unlike natural shifters, any change by a created shifter carried the risk of loosing oneself in the beastial rage, with a greater chance during forced transformations. 
As the Changeplague spread among the Shifters of Uthrangir the Abobnelyths used their new soldiers to attack and transform huge numbers from the human tribes recruiting whole communities into their armies. With the increased forces Sinastric would be able to destroy Nagusacktem. 
While this was likely an unintended result as it was not needed for the Changeplague's original purpose, the infection's ability to be spread by non Shifters such as humans and elves transported the contagion far beyond the shores of Uthrangir, so that it has reached all corners of Therrmordia and all known Shifter populations. It is possible that there exists communities of Shifters not affected by the Changeplauge, but but such a group would have had to be completely cut off from all humanoid contact for millennia. 

Terminology

The Changeplague refers to one of two closely related conditions regarding shifters. The Changeplague infection allows the transfer of some shifter abilities and disabilities. The actual transfer of those abilities is known as the Shifter Curse, and those affected are known as demishifters or shapecursed. A Shifter that was not infected with the Changeplague would not pass on the Shifter Curse to a bitten human. 

Transmission

The Changeplague is extremely virulent. It is present in both the blood and sexual fluids, as well as being transmissible by aerosol. Simple spending time around someone who has it will pass it on. While Shifters are the only ones effected by the infections, non Shifters can be carriers of the Changeplague, and due to how easily it spreads and its blood transmissibility almost all mammalian non Shifters are born carriers and other non Shifters become carriers soon after birth (or hatching, or whatever). 

Trivia

Mulephicent, the Abobnelyths who created the Changeplague and, indirectly, all shapecursed, would eventually have its consciousness destroyed and its essence consumed by the descendant of a shapecursed, Solificus.

Type

Magical