As seen in movies, there are various types of zombie virus and reasons humanity is fighting for survival.
So, how could it have all happened?
Medical: A Vaccine goes wrong, creating the zombie virus.
Military: Experiments to create a 'super soldier' go completely wrong.
Space: either a returning space exploration satellite or a meteor contains contains something nasty.
Corporate: a company creating something that is the "benefit of all man kind" (maybe the secret to eternal youth), fitness/ energy drinks and so on, and gets it all wrong.
Terrorist: some sicko wanting the destruction of a nation or maybe all of man kind creates a virus.
Eldritch Horror: an ancient horror/spell is behind every thing.
Monster: some kind of life form takes over the victim by entering the brain.
Nuclear: a nuclear reactor blows up, or the power company dumps barrels of nuclear waste, both of which cause the zombies.
So, do I chose one of these, or have it random and the players have to discover it for themselves? Maybe most of the rumors are false, where as one of them is actually true.
"No More Room In Hell" has charts that allow players to discover why it has happened, and can earn survivor points for the discovery, but that is as far as it goes.
Using this chart I am thinking that when the players earn enough points based on the finds then a cure can be created... provided they can find a survivor that allows them to do so, such as a microbiologist. This specialist can be found only by meeting survivors and getting the right one needed on a random table.
Maybe each piece of information gains one point towards finding the reason for the outbreak. Once the players find 100 points worth of evidence then they can begin to work out the cure.
I will have to put some thought to it, and when I come up with the answer I will post it here.