For any living being to survive, they need a life force. The battery for this life force, commonly known as mana, is a finicky thing. Few humans have a deficit, and even fewer an excess, known as acrits and espites respectively. More often, humans have a completely balanced amount.
That doesn't mean that it's any less sensitive. Even the slightest altercation to the mana's flow without being an espite will cause large, green sores, high fevers, and eventually, death.
After the sores form, they grow. They grow, the skin stretches, then they pop. The mana inside evaporates in contact with the air, mixing in and infecting it. That usually wouldn't be that much of an issue.
There was a quarantine facility set up for just this purpose. However, if they are outside of the quarantine facility, the mana contamination spreads. Inhaling the mana leads directly to an excessive amount in the system after all.
People had taken to wearing gas masks designed to prevent mana from entering their lungs. They added stronger filtration systems to buildings, something expensive enough that only the extremely wealthy could afford to live alone.
As these changes came, society adapted. More fanatic religious groups formed. Schools became some of the safest places to send your children, but cost an arm and a leg without someone taking special interest in you.
The death rates skyrocketed, and people looked for answers. Some people started looking a little too hard. Groups of espites were found. These races and afflicted practically radiated mana. Humans started to grow angry. They hated that the so-called monsters wouldn't be effected by the disease picking them off one by one.
The humans made effort after effort to find a way to become one of them. Their efforts were in vain, so they separated these groups from them. Given barely any resources, these groups had to find ways to live. Of course, the espites eventually found ways to weave their way into human society.
The races, those born with the mana, were placed in different categories. They were vampires, serpents, nymphs, elfs, demons, dragons, and ancient ones.
The afflicted were those who's beings had been warped to accept the mana. There were the seers, alchemists, cursed, fireborn, sorcerers, star dancers, and mages.
Each of these espites had found a way to survive. All but the ancient creatures and ancient ones who remained unknown were nearly thriving. The elves and nymphs became relied on for farming and air filtration. Alchemists made new, more efficient masks. Vampires became a pinnacle of wealth and a powerhouse for trade, though that was fairly conflicting because of the whole "they drink blood" thing.
But the humans still couldn't accept them. They could deal with the people who looked like them, act like them, but just aren't them. Too similar to be different, too different to be the same. That's how it would always be.