Heres my theory,
Scratch is the source of the infection. But first a little back-story.
Scratch use to go out with a guy named
Bill. Bill was your normal clean cut, sane, intelligent guy. But she was so bat sh*t insane/sadistic, it drove him crazy. First he shaved off all his hair. Then he started getting tattoos all over his body. Then he started killing people. Hay you would too if you had a relationship with Scratch.
By the way
Bill Roberts gave Scratch her nick name, because thats what he did after they had sex...
Yup. You guessed it. The zombie virus is an STD. Thanks Scratch.