In my game I propose one theory on hour our forced evolution or abuse of cloning results in mutations in our dna resulting in the abnormal behavior of anyone that's been cloned too many times. I also try to reflect how people who have never been cloned react to those that have been. This is one potential however it's nowhere near the more frightening concepts I imagined. The reason I settled on it was the message, telling a story and following tradition of what a good zombie story is supposed to be. What I really think would happen in a real zombie outbreak is much different.
I see it coming in one of two ways. A parasite that easily can live in all kinds of species and can survive in water alone. These parasites would rapidly infect and multiply spreading in ways we couldn't hope to contain. Eventually it would be nearly impossible to eat or drink anything natural at any time and potentially dangerous to be out in the rain from an infection standpoint. It'd be likely that these parasites would also be easily transmitted in saliva and blood making them that much deadlier as then you have to worry about what you touch and use, and how you use it. How it would really effect you or change you into a zombie is a good question though. I would imagine a rudimentary intelligence that builds the more of the parasite is in your system until it gains control. In order to keep you alive for it's own survival and propagation it would likely find ways to repair damage and keep vital organs working and would do so quickly making killing a parasite based zombie very difficult indeed as it wouldn't be a centralized intelligence where a headshot would easily kill it I would imagine.
Then there's the traditional virus. Airborn, transmittable via water, cross species, this relentless pandemic would sweep the nation before we could possibly hope to control it. Though it could just as easily take a much more subtle approach. It starts off small, some land locked country far from regular civilization in an area prime for bacterial propagation and evolution. It infects but leaves no sign it's even there. It then begins to spread, it's months before doctors start noticing the abnormality in blood work. They begin to analyze it and find a treatment. It slowly evolves or mutates causing minor loss in body fat. A group effort is formed once it becomes apparent the world is rapidly becoming infected. Some of the more paranoid nations start restricting travel into the country. Then in some highly overcrowded city like kowloon it mutates again. Within the first week major deaths are reported and substantial weight loss for the survivors. The mutation now eats away at brain cells causing loss of fine motor control, hemorrhaging from every orifice, constant vomiting, dehydration, liver failure and in some cases blindness. The first wave of infected starts to turn on fellow humans and violently tear them apart. The human physiology tries to adapt and survive some partial immunities keep some of the infected alive though with their diminished brain capacity they're nothing but mindless drones prone to extreme violence and cannibalism. At this point most countries have completely disallowed all traffic and have declared martial law. The borders aren't protected but the blocked streets slow the spread. Governments start handing out bottled water, killing rodents, pests, and other creatures, burning everything.
During the second week the first UN meetings on the subject are held. Consideration is given to joint military operations to eliminate the infection in the most heavily populated areas while also establishing perimeter control until a cure can be found. One of the greatest collaborations in history takes place as the worlds greatest work around the clock to find a solution. By the end of the week the first strikes begin. By the end of the first month military action is failing to control the spread and it is now confirmed every country in the world is at least 10% of it's population dead, and at least 40% of the remaining population is infected. Some of the first countries are declared no mans land. In the following weeks the first nukes are used on countries deemed devoid of life. The containment perimeters on existing locations are expanded and extreme measures are taken to maintain them, including massive terraforming, constant bombardment, and automated turrets that fire on anything that moves.
After 3 months the collective collaborations of scientists has come to a conclusion that no cure can be made and the project is terminated. By now over 60% of the world population is infected. Constant nuclear strikes are authorized and the first centralized evacuations occur to relocate the population to an area that can be safely controlled and maintained. Anyone who is infected has a sample of their blood taken then executed on the spot. A few scientists still hold out for hope of a natural immunity. Experiments are conducted on some of the infected to find deterrents, efficient methods of killing, and survivability under extreme conditions. During the relocation process it's estimated the infection spreads to nearly 80% of all remaining world population.
The new underground cities prove effective at preventing infection. Massive air filtration, and cleaning processes that are isolated from the outside world. Daily testing of blood samples the test has been honed down to a process that takes only seconds for extremely accurate results. Positive testers are isolated in case of false positives and tested for potential immunity. The food supply is stable though not of the highest quality. Water is almost too clean though the supply is tightly controlled, maintained, and monitored. Alcohol becomes the standard liquid of choice due to it's relative safety aside from other less practical reasons. The supply is abundant. The outside world is almost completely lifeless and all military efforts have been abandoned in favor of protecting the last city.
One day some how some way the virus makes its way into the city. By now it has evolved several times and is now hundreds of times more infectious and deadlier than before. hundreds are infected within minutes, within hours the greater majority of the population is now infected and the first signs containment have failed start to show. By the end of the day there are fewer than 100 people left in the world and 20 of them are confined within the city now overrun with the infected. Perhaps some of them are actually immune, perhaps they're just lucky. The species is on the brink of extinction and nearly impossible to propagate under the current conditions the end of all life of the planet is now imminent. The infection has greatly altered all other life on the planet and the atmosphere has adjusted accordingly making life that much harder on the survivors.
At this point either the human race dies, or a glimmer of hope is revealed as the few survivors begin to rebuild and have offspring. The world will never be the same and the human race will be starting practically from square 1 if it survives. It's likely the virus is alive inside all the survivors however doesn't effect them for some reason, likely immunity possibly something else. Over time this causes a mutation in the genetics redefining what a human even is.
That's how I imagine a real zombie apocalypse happening, because even if you stop the people at first the disease will find a way to travel and once it's spread everywhere there's no stopping it.