Old Yharnam & the Ashen Blood Plague
The Plague and How it (maybe) Spread
The name of the plague, “Ashen Blood", that struck Old Yharnam comes from the greyish, pale appearance of an afflicted person's blood. One observation we can make is upon striking the Blood Starved Beast in the Chapel of the Good Chalice, the blood is a gray color.
Game notes state the Ashen Blood plague triggered the spread of the beastly scourge (sometimes referred to as the Beast Plague), but never directly explains how.
A common theory is that the Healing Church introduced the populace of Old Yharnam to an infectious agent in their research into the Old Blood and the Great Ones, resulting in the Ashen Blood plague. Below are some of my thoughts on this theory:
The grayish blood of the sick and the Church's interest in "paleblood" is quite the coincidence. Moving forward with the idea the Healing Church used Old Yharnam's citizens as research subjects, the Church may have thought that the pale color of the Great Ones' blood (which we can assume they discovered during the events at the Fishing Hamlet) was a key component of ascension - or becoming a Great One.
Their logic might have been that the blood was key to facilitate a transformation - or it may have been an exploratory venture into the biology of the Great Ones. Regardless, once the people began falling ill, and with the antidotes only providing temporary relief, the sick may have sought blood healing.
As for the causative agent of the Ashen Blood plague, I personally believe it was derived from snake venom.
Snake venom, in some species, can destroy red blood cells (RBCs) through a process called hemolysis. Since RBCs are what gives blood its color, the assumption is that blood devoid of RBCs would be pale (the remaining blood products, like plasma, are a pale color).
But, hemolysis causes the contents of RBCs to leak into the rest of the blood. This is often seen in phlebotomy after a blood sample is collected in a vacutainer tube treated with an anti-coagulant. Once in the tube, the blood will separate nicely in layers unless hemolysis occurs - then the blood components will unevenly separate or "mix".
I say all this because, if a person is exposed to venom with hemolytic properties, their RBCs would rupture inside the body and not result in pale-colored blood. Their blood would likely still be pink if not reddish, not fully pale or pale yellow.
I'm being a stickler about this observation (the numerous other medical inaccuracies in this cosmic horror, fictional video game setting notwithstanding…), but there are other thoughts I have on this theory:
1.) Healing Church involvement in Forbidden Woods:
There is a white church hunter on a thatched roof in the Forbidden Woods, and there are Church Giants in a secret passage linking these woods and Central Yharnam (…a ladder leads up into the graveyard near Iosephka's clinic). While the giants could be there to keep those from the Forbidden Woods out of Yharnam, they could also be there to provide safe passage for Healing Church members traveling to and from.
You use the same antidote to cure yourself of slow/rapid poison following attacks from the snake-men and snakes as you do the poisoned beasts in Old Yharnam. While Bradford Hill's (commonly "Hill's") criteria for determining epidemiological causal associations is not perfect, it helps here.
3.) Thematic inspirations:
The science of Bloodborne appears greatly inspired by research of the mid-1700s to the end of the Victorian era (1901). To name a few, we see the influence of Galvani (medical electricity), electromagnetism, and the study of optics (as seen in the Nightmare Lecture Hall, and by the existence of glasses). Victorian era doctors believed snake oil could act as a pain reliever and could cure ills, despite most of its use being a hoax. Not only is the use of snake products a historical tie-in, but it it also another false remedy which seems on-brand for the Healing Church.
As for how the people of Old Yharnam were exposed to the infectious agent that led to the Ashen Blood plague, there is less material to work with. One theory is the use of the water supply as the mode of transmission, and I can't help but gravitate towards it. I'm not suggesting they milked a million vipers and dumped the products into the water, but again, there is little point in holding tight to real science in a game taking inspiration from Lovecraftian fiction.
A common-source infection (such as water) suggests they wanted it to spread, but not between people, and for it to spread quietly. Also, poisoning means a delayed or slow death, leaving plenty of time for the Healing Church to study the sick (and observe potential compound effects of blood ministration).
Evidently, things got out of control with the infected, and the Healing Church had hunters burn and abandon Old Yharnam. This stops the spread of the Ashen Blood plague, and allows the church to wash their hands of the situation.
Snake oil salesmen and charlatans, all. At least here the frauds selling you cures for your ills under the guise of religion actually value scientific rigor. Silver linings?