The Mutation Virus originally came from the infamous biological company called Ramay Chemical, which was basically a company gone mad. The company had originally been funded by the US government, but after a series of controversial happenstances, they forced the company to shut down. After a long lasting investigation concerning the company's actions, researchers discovered the use of human test subjects, mutated in various, sometimes hideous ways. The original owner of the company was arrested and sentenced to death row in prison.
It didn't end there however.
Ramay Chem went underground, creating unstable drugs that caused mutations of any kind on those that ingested them through the use of drug vials. It was similar to normal drugs, but very much chased after by the government, like a witch hunt, only found and sold at the black market, at high prices.
When Ramay Chem's hide-out was found, the last people behind the mutations were eliminated, but not before one of them hit a death switch.
Agents discovered it only days later, that there had been a viral breakout. After the incubation period was over, and the virus had already spread too far to be able to control it, agents behind the operation of the elimination of Ramay Chemicals, fell ill.
Their bodies changed in ways no one could ever have encompassed before the creation of the name of the infamous company. They were mutilated by their mutations, sometimes developing growths on their bodies rendering them unable to move. Other people's transformations were cleaner, much cleaner, creating fully functional appendages, or causing inhuman hairs to sprout all over their skin. Others looked like any other human being, but had changed drastically from the inside, or in places where most wouldn't look. There were sometimes mental changes that came with the mutations as well.
The virus slowly spread over the populace.