Let's Pretend: If It's Not a Virus, What Else Could It Be?
Let's get creative...
What if Radon Labs (or someone nearby) was in possession of an alien (duh duh duuuuuuh!). They were running tests on the tissue and someone in the lab was contaminated. The alien blood/tissue could have acted like a virus. A pretty nifty defense--if you bite me, I colonize you.
Theories on how the virus/bacteria/infection/mutation started. Zombies?
My guess is military experiment but that just a guess. Only saying so because of the tatoo prisoner guy he may have been in that institute and they infected like he could be the center. Oh and how could these "things" be zombies zombies are walking corpses therefore they would just fall apart and rot. these dont.