The above picture is taken from an old project of mine. The goal was to build a simulative model in order to predict what humanity's fate would be, long-term, in the event of a zombie apocalypse as seen in classic films like those of George Romero. It was actually my final project in a high school class that was based around using a computer program called STELLA to model these sorts of complex situations. Prior assignments had modeled things like the spread of a disease and the populations of wolves and deer in a wildlife preserve; part of my interest in putting together this particular simulation came from how it mixed elements of those two prior models. You have predators which infect and convert their prey. How would such a premise logically play out?
The simulation takes into account more factors than just the method by which the virus spreads. The two major things I wanted to take into account beyond the basic model were supplies available for salvage and survivor's experience in dealing with zombies. If there's a ton of abandoned houses around with fully stocked fridges, an average survivor has lower odds of starving or freezing to death. If a person has never had a first-hand encounter with a zombie before- knows nothing about the situation they're in except that there's a moving corpse in front of them- then their odds of surviving that encounter are going to be alot lower.
The variable in my simulation, then, was the size of the initial outbreak. If the dead rise from their graves all over the world, and society has collapsed by the end of the week, people are going to be less prepared than if they've had a couple weeks to watch news of nations in other hemispheres slowly crumbling while wondering if they'll be next.
The model's results are interesting to me in three different ways:
-The single scariest factor is a way in which the zombie-human factor does *not* match the typical predator-prey relationship. Cruel as it is, that type of system has inherent balances; wolves will start starving to death if they kill and eat 90% of the deer population. But zombies don't starve. They get along just fine even when there's only one of us left for every ten of them.
-As you can see, the variable- the size of the initial outbreak- leads to some interesting variations in the model. But even the smallest outbreak- only 50 initial infections- still leads to the death of over 80% of the population. However, the only scenario in which humanity is actually wiped out- leaving only a population of slowly eroding zombies- is the one where a full quarter of the population is initially infected and turned, so that when only the seasoned veterans remain there's just too many zombies left for them to handle.
-Lastly, the model ignores/handwaves a certain factor which is rarely addressed in zombie fiction (Though Max Brooks made a concerted justification effort in World War Z, bless his heart): There is NO WAY the virus is going to spread effectively if it's only transmitted from a zombie to a living person's bodily fluids. Because the world's just too big. In zombie fiction you can drop into some random point in the wilderness and there'll be a hundred zombies in a mile radius. The virus will spread across the country like wildfire. But in practice. . .how does a zombie make its way across open countryside to another major population center? Romero's zombie virus, as presented, just isn't cut out for global armageddon.