There are five main types of affliction: Spawn, Chimera, Lurkers, Hunters and Beasts.
Spawn are the most simple and are in fact the larval form of the other base types. Most crawl into an orifice and begin devouring their host, starting with non-critical areas and spreading to other areas, eventually settling in the stomach and growing. The host often dies in agony, the spawn bursting forth into its adult form. Most victims are host to multiple spawn at once, with the largest spawn eating the others to gain mass.
Chimera are creatures with a singular goal: infect a human's brain. Often, a Chimera will release spores to lull the host to sleep, before crawling into the mouth. There, they burrow through the esophagus into the chest cavity, wrapping itself around the heart and then sending tendrils to begin linking with the host's nervous system, hijacking their bodies. Then they will take control of a human's reproductive system, replacing sperm and ova with incubating spawn to be released during copulation.
Lurkers are ambush demons that prefer to hide in dark areas and ambush anyone unfortunate enough to be nearby. Often, a lurker will grab a human and consume most of it, but leaving the head and planting eggs inside the brain to grow spawn. Lurkers are the reason that all dead patients have their heads removed and burned.
Hunters are straight forward but deadly creatures. Ranging in size from a foot long to over ten feet in length, hunters attack anything and everything, from small rats and mice to dogs and cats to people and livestock. Hunters reproduce by capturing a creature of appropriate size and impaling them on an ovipsitor, before being placed inside a special pouch where the young hatch and grow, before emerging. Hunters are mostly cannibalistic but have been seen hunting in packs when they fear retaliation from humans.
Beasts are rare creatures that tower above the landscape. Female Beasts stand a minimum of 30 feet tall, but actually fly on large wings, gliding in the air currents overhead. Male beasts, however, stand only 6 feet tall, and their shorter wings are made for dive bombing. Several males, eager to mate with a female, will dive upon human settlements and haul them up into the air. Female humans are stripped of their clothing and offered by the males to the female, who may choose to use the human for incubation. Male humans are simply consumed.
More monsters may be added later as mutations occur
It was not December 21st, as so many tried to predict, but rather July 9, 2014 that was the official date of the end of the world. An infection began in a small South American village, which quickly began to spread. Doctors tried to treat the outbreak but only perished in gruesome deaths. As the infection spread by people fleeing for their lives, countries began going on lockdown, killing anyone who showed signs of infection, but it was too late. The infection rolled over millions, sparing only those who hid in churches and sanctuaries. Believing this to be a sign from God, many began to become devout worshipers, though anyone who ventured outside soon died. No one was sure why, but only churches were spared. Eventually, lack of supplies and a need to investigate caused the church residents to begin exploring, fighting demons with whatever they had available. With most of their food dead, many demons dropped dead of starvation, leaving only the hearty to survive.
As the world began to partially rebuild, the churches began to unite, eventually finding a way to reach the Vatican, and forming communities surrounding the churches. Under the guidance of the Pope, the churches began to take on more significance than simply a place to find shelter from demon attacks. Cults sprang up, some being consumed by demons, others simply existing, worshiping the churches. Fearing that the spread of false prophets would lead to confusion, panic, and death. With a holy edict, a Crusade was launched to purge all who dared to oppose the Pope and ignore his rightful place as the ear of God on Earth.
Over a century of fighting between small scattered communities left most of the central areas of Europe and the eastern portions of the United States under the rule of the Pope, while those beyond became heretics. The Children of God continued to grow, eventually finding vast universities where the knowledge was preserved in books that let the Children build more and more. Eventually, the Centrale University was founded, built to study the demonic threat and teach others how to combat their lethal foes. As a center of research into eventually killing all demons, the University is under constant guard. Only way in or out is either as a patient or as a professor/doctor trying to study and purge the threat.