It has existed since man could understand the concept of pain.
It has existed since man could suffer.
Many ancient cultures have very diverse myths about the creation of the world, life and human purpose. The concept of disease, however, always seems to be pinned on malevolent spirits or demons. The Nosoi, the Guul and the Mambabarang are a few examples. As time carried on, these were dismissed as silly tales from "primitive" cultures. However, that only allowed for it to move more freely. If people aren't looking for it, how could it be inhibited? Usually with spirits or gods, they wither and die if they are forgotten. This creature is quite the opposite. It fares better if no one is looking. It delights in our attempts to control its handiwork.
The creature has changed vastly over the course of its existence. The form it chooses changes on a whim, depending on how it feels like terrorizing the population. Sometimes it appears as a human skeleton. Sometimes it wears a mask, like the Plague Doctors of old. It has been known to take the form of animals, sometimes just the bones. However, the appearance it chooses doesn't matter. The only ones who ever see it are usually artists or the mentally unstable. No one believes them. It will only show itself to those who it deems worthy.
Pray you are never deemed worthy.
It is not only the origin of disease, but also its guardian. It creates pandemics for fun, letting them loose upon the world, laughing at the hysteria and panic it creates. It ensures that disease is never stopped. It helps them mutate, resist vaccines, and grow stronger. Do you know how many diseases have actually been cured? One: Smallpox.
This nasty creature is behind most of history's greatest pandemics. The Bubonic Plague? It was. Yellow Fever? It was. AIDS? That one was actually the US Government, but you can bet the creature wiped a tear of joy from its eye and smiled, proud of how humanity has learned from its techniques.
So, while modern medicine does everything it can to keep the creature at bay, while not even acknowledging it, it smiles and continues to turn the natural wheel of mortality. The more diseases we try to fix, the more it will rain upon us.
There is only one way to be born, yet over a thousand to die.
And now you know the reason.