Yuki is a curse entity that was originally created to punish evil, she now infects anyone that suffers from traumatic events due to a mistake they may have, may have not done..in her corrupted way of "doing whats right". But due to the large numbers of people falling to the curse.. this made people aware something is happening..something wrong...Civilization has no idea what exactly is causing this.. most assume its some sort of Disease/virus, others go based on their beliefs, while others refuse to believe it's even real.