not the gov. those pussies don't even like genetic research... private company looking for big money though... not with radiation either, we already figered out that that shit doesn't create huge ants and spiders either. also no voodoo shit, or other religion-linked stuff, that shit will never happened. same deal with aliens. my guess is a man-made fast-mutating virus (either accidental or on purpose).
Have you read World War Z? If you follow the premise set in that book it would appear the "plague" has always been here at different strengths. It just occurs naturally. I suppose some kind of catalyst is needed. Maybe when the earth reaches it's boiling point and can no longer handle the numbers it releases the catalyst and kills off populations. Maybe when the amount of humans goes beyond a certain number the virus activates in order the dwindle the population back to something more manageable. It could be some crazy dictator developing the super weapon that is accidenlty released. Or it could be a cure gone bad. I guess the individual has to decide. Hell, it could be every scenario.