I'm not sure if anyone has mentioned this but Shane was acting more schizophrenic than usual before his death. I'm thinking it was a symptom of being infected by the airborn or contact strain of the virus. Sure he was a nut job to begin with but something exasperated it.

The real question now lies in how contagious this thing is and what the symptoms are if you contract it. Unlike a bite it does not appear to kill you quickly and then zombify you. Its more of a gradual progression of mental illness followed by zombie-ism after death or possibly also after a long incubation period.