Someone came to town

Well, that was probably the freakiest thing I've seen. I was tracking a certain someone, then happened upon an apartment building. I entered, only to find that everyone was dead. They had sores everywhere, and the couple people still alive were busy coughing to death. There was a symbol sketched onto the wall that I don't know.

As I watched, veins of some black material moved through the walls, almost as if the wood of the walls were getting diseased, and they started to crumble. I got the hell out of there as fast as I could. Hopefully I didn't catch anything.