They have cures for the vampire virus for when a patient is newly infected. They tried it on Yuuto, and it didn't work. Yuuto's case mystified the doctors. It later turns out just a little bit into the common route that he's a "vampire devourer," a very special kind of vampire. I haven't actually read any single route to completion yet, though, so I don't really know how or why that actually happened, or if it's ever explained at all.
edit: Okay, so, @Justin579, if you want the answer to your question, you'll have to wait until the full Miu route translation is released. The answer lies within that.