"Can't you feel the rot in your own body? I can. Every cell bursting as your immune system desperately tries to fight it off...the infection...the pus building up. The open sores and failing organs. Your mind may be strong, little runner, but your body? Oh, your body is mine. It has been mine for quite...some...time."The Plague Doctor is the Fear of Illness and Disease. Not those of the mind, but the body. He rejoices in plague and illness, influenza and pneumonia. The Plague Doctor is disease. Pestilence, plague, illness, suffering and pain. Every smallpox victim has cursed his name, knowingly or not. Every cancerous tumor is a choir singing his praises. Modern medicine is to him an evil, and every year he does his best to stop the spread of medicine and vaccine.
For the Plague Doctor does not merely cause illness, he actively spreads it. Often his victims are not doctors, or even people scared of being sick. Often his victims are politicians or other people in power. If people were to go to aid nation rife with ebola, the Plague Doctor would extend his influence to that nation's leaders, slowly convincing them to reject the aid. To let the plague spread.
He will go after doctors, making them misdiagnoise and give the wrong medication, or to the common citizen, convincing them that the lump on their arm? It's cancer. That sniffle? Scarlet fever. His victims go to the ER or to the walk-in clinics, flooding the hospitals with people who have nothing wrong with them. This way, by crowding the medical facilities, doctors cannot treat the legitimately ill.
If there is one medicine that the Plague Doctor does support, it is quackery like crystal healing or homeopathy. With no science behind them to say it works, only the belief of people willing to spend money better used on antibiotics, the Plague Doctor supports it. When people with curable illness die because they trusted a crystal to heal their infection, the Plague Doctor rejoices, filled with ecstasy.
The Plague Doctor goes after all who could spread plague. Those who somehow escape his plague find themselves forever paranoid. Forever believing they are sick, and constantly having headcolds or fevers. There is no true escape from the Plague Doctor, save death.
Particularly unlucky victims of the Plague Doctor find themselves inside his realm, his pocket dimension, The Crumbling Castle. It is a horrible, living castle, constantly crumbling and falling apart, revealing bloated, pustular, breathing flesh underneath a thin stone covering.
The Plague Doctor sometimes comes to medical professionals, nurses, doctors, psychiatrists, that he has been influencing for some time. Often, they have lost their degrees, their right to practice. He comes with an offer. "Join me, and practice medicine with no laws telling you what you cannot do. Join me, and be free of your past." Should the person take this offer, they become a Oath Breaker. A servant of the Plague Doctor, devoted to spreading plague and illness forever and everywhere. Should they refuse, he will kill them with one of his multitude of strange medical devices.
When he comes to these people, who may also be runners of his, he appears as a plague doctor of the middle ages. His form constantly drips pus and his voice is creaky and garbled by infections of the throat. Behind his leathery mask, no man or woman alive knows if there is a face or not. Some believe it is his face. From the robe he wears, all sorts of horrible tools can be produced, rusted old medical devices from an era before anesthetics or sanitation.