The Institute of Experimental Medicine messed around with cholera, tetanus, and, holy s**t, even the bubonic plague. They primarily experimented on horses -- which you may recognize as the official disturbing animal mascot of everything from The Godfather to The Ring -- with the end goal of producing serums from their blood. And, of course, several of the researchers themselves contacted said plague.
The universe? She does not enjoy being taunted.
The Institute of Experimental Medicine was run by a Russian veterinarian named Tartakovsky who, of course, was also one of the folks to contract the plague. So, we've got mad scientists, tortured animals, and plague victims all wrapped up in one perfect supernatural package. Oh, and all the corpses of said plague victims were burned right there at the fort, because history is the best horror author in existence. The only thing that could make this a more perfect setup for a horror flick is if the fort had been abandoned, and, now, a bunch of stupid teenagers were holding parties there.