Precisley why I said I would explain things more in the OOC. Welp. I'll most likely work on it now. 

To answer your question, I might actually change that. I myself don't agree with it. I'll have it to where a nuclear plant had a spill, like radioactive sludge, and the gases from it affected the workers. Turned them to madmen, then into a mutant creature. The infection can be spread through being bit or going near the radioactive area. 

The once workers move into the city and start attacking people.