I think the show's creators have been pretty upfront about the show not being about what happened during the apocalypse, what caused it, finding a cure, etc. Their take is that it will always be about the survival afterwards. Even when they were with the guy at the CDC, not too much was given as far as when, where, how everything kicked off, just that it was everyplace and as far as that guy knew noplace was left untouched. Even the mention that the French equivalent to the CDC had said they were close to a cure before they went dark has been pushed aside (which makes sense, not like it would be easy for the group to head over to France and try to find the place, let alone understand any of the leftover research unless they had some scientists along with them).