Tobias knew because he was following every news story on the Internet, every radio report probably. He's a computer geek - he tutors his classmates so he's bright and he listens to things no one else pays attention to.
Jenner didn't know what caused the virus and I'm sure neither will anyone know on FTWD...this new show is supposed to be about when it started...but if no one in TWD has all the answers, neither will these folks.
