Future Press game guide description: "Iosefka is an earnest, kindhearted doctor of the Healing Church who oversees the clinic in Central Yharnam. She cares deeply for her patients' well-being and strictly forbids you from re-entry into the facility for fear of exposing them to the scourge, but is still willing to provide you with her valuable experimental blood vials."
Iosefka is a doctor for the Healing Church who oversees the clinic known as ‘Iosefka’s Clinic’ in Central Yharnam. After the player leaves the room they wake up in and reload the area, Iosefka will close and lock the door behind them, seemingly as if she was waiting for them to leave.
The Clinic Iosefka operates is where foreigners to Yharnam and patients come to receive Blood Ministration. Yharnam is the home of blood ministration. The Healing Church controls all knowledge on blood ministration, and all varieties of blood. Once a patient has had their blood ministered, a unique but common treatment in Yharnam, successive infusions recall the first, and are all the more invigorating for it. Blood ministration is the pursuit of communion, and involves the transfusion of unknown blood. Visitors to Yharnam often come to the city to receive this blood as a means of treating whatever form of sickness they have. Before receiving the transfusion, patients must sign a contract with their Blood Minister, though what is included in this contract is never explicitly revealed.
From this information it is clear that Iosefka is affiliated with the Healing Church and her role is to provide visitors to Yharnam with blood that will heal them. But there is certainly more to her than that. Iosefka is a special Church doctor, and therefore a specialist in experimentally backed blood ministration and the scourge of the beast. She is well aware that the blood she is giving to her patients will have some seriously adverse side-effects, and this is a possible reason as to why she will not allow you back into her clinic after your blood has been administered to. She will tell you “the patients here in my clinic must not be exposed to infection” though as a specialist of blood she must already know that it is the blood that causes the infection. Iosefka knows that the hunt drives hunters mad for blood, turning them into beasts, and she therefore does not want to risk her research being brought to an end by some bloodthirsty hunter. If you attack her door she will eventually say: “You are no more than a beast. I should’ve known…” indicating she knows what happens to hunters. She is seemingly sympathetic to the player’s cause though, and will give them vials of precious blood.
So what is her true agenda? Iosefka is seeking a cure for the scourge, just like any good doctor would. Her method involves a slow and careful blood refinement process, which was likely devised by Iosefka herself. We are not told what Iosefka hopes to achieve from refining the blood, but there are some likely conclusions to draw. Iosefka knows much about blood, and what she hopes to refine from the blood is the blood borne infection that causes those blood is ministered to, to succumb to the Scourge of the Beast. When we look at Iosefka’s blood vial we can see that it is not red but rather an amber, light yellow color, and yet it is called blood and its affects are even better than regular blood vials. Iosefka’s vials are “highly invigorating” and restore a larger amount of HP. Evidently Iosefka is on to something. Because of the coloration of the blood we can make an educated guess that she is refining it of the red blood cells leaving just the plasma. This quote from The Paleblood Hunt (https://docs.google.com/document/d/1JL5acskAT_2t062HILImBkV8eXAwaqOj611mSjK-vZ8/edit) by Reddit user Redgrave explains the significance of this succinctly:
“I am not a doctor, nor will I pretend to be. As such I am likely vastly oversimplifying things, so take my explanation with a grain of salt. Blood consists of three distinct ingredients. First are Erythrocytes, typically referred to as Red Blood Cells. Red blood cells travel through the body of a human being and distribute oxygen to the body's tissues. Red blood cells are the primary means of life support and nutrition for the body's tissues. Second is Plasma. Approximately half of blood is plasma, a liquid that serves as means of transportation for the blood cells which carry out their task in supporting the human body. When blood is centrifuged to separate the ingredients, we find something very interesting about plasma. Plasma that has been separated from clotting proteins and blood cells is actually an amber, light yellow color. This liquid is referred to as Serum, or sometimes pure blood as it is the liquid blood in itself without any other ingredients. The third ingredient of blood are the Leukocytes, or White Blood Cells. White blood cells exist as part of the human body's immune system; they attack infectious invaders, foreign substances, and clean after old and dead cell structures.
Blood is such a central theme in Bloodborne, everything revolves around Blood, even the title. Especially the title. Bloodborne. A bloodborne pathogen, a disease of the blood, this is what the Scourge of the Beast is. It's an infection, a disease that spreads from person to person through the transfusion of blood. Master Willem sternly reminds Laurence that: "We are born of the blood. Made men by the blood. Undone by the blood." The scourge infects the victim by attacking and tainting their blood, corrupting them.
Now look at the Kin, ascended mortals who have become Kin of the Cosmos. They have escaped the taint of the Scourge, moving past it. Indeed they bleed a clear, amber liquid; serum, to be exact. The Kin of the Cosmos have cleansed their blood of the tainted scourge, separating it from the other ingredients and becoming pure beings.”
Redgrave extrapolates that the blood that is a clear a amber, light yellow color is the blood of the Kin of the Cosmos. Whether Iosefka obtained this blood through contact with the Choir or Ebrietas or whether standard Yharnam blood that has been carefully refined actually is kin blood, is unclear. But Iosefka appears to be coming close to a vital breakthrough. If her work was allowed to continue it may have led to the ending of the Scourge of the Beast, as the blood she could treat the patients with would actually be kin blood, which is cleansed of the beastly taint. Unfortunately, Iosefka is removed from the game early on in the night and replaced by an imposter. Entering the clinic from the back entrance, the player finds a Celestial Emissary standing where Iosefka used to stand. When the player kills this emissary it will drop one of Iosefka’s Vials, bringing to a close a shining light, in the dark of the hunt…