It's actually a disease caused by a toxin, a toxin that is the byproduct of a parasite attempting to mass reproduce. But hey man, nice shot ;). With all of that being said, anybody who was an adult at the onset of the outbreak is probably dead or rather old (70+). There's an entire generation that was basically born during the end of the world. Which is actually shown to be a huge side effect of people being holed up in a building with no power and no entertainment. Things start happening in the dark. I'll probably prepare a timeline now that I think about it.