Zombies are a major part of media, common enough that the word “zombie” instantly brings visions of creatures in some mass apocalyptic scenario who spring up overnight and leave a huge part of the population as mindless fiends, roaming around in search of other humans to infect. Zombies are simply terrifying, and movies like Train to Busan or World War Z make us fear the possibility of a zombie pathogen. Although real life diseases can take control of a large part of an organism’s body, and epidemics can spring up seemingly overnight, the combination of factors that would cause a zombie apocalypse are nearly impossible for a number of reasons.
First, let's classify exactly what constitutes a zombie virus. In media, zombie viruses are often shown as taking over an organism’s mind rapidly after it is bitten or dies, causing the organism to violently attack others. There are a multitude of such parasitic organisms or viruses that appear to cause animals to attack others in an attempt to spread the pathogen, however, these parasites merely encourage and exacerbate the organism's behavior when ill, and don't take control over the organism’s mind at all.
The rabies virus for example, is probably the closest thing to a zombie virus in mammals because of the way infected animals act. It’s common to hear of rabid animals transferring the disease to humans or other animals through bites, much like a zombie virus might. Yet, the rabies virus is nowhere near as potent or as much of a threat as any of the fictional viruses. This is because the rabies virus only causes pain and discomfort in animals as opposed to actually controlling them. The pain and discomfort causes them to become defensive and confused, the way your family pet might, when it's injured or sick. Rabies takes advantage of this behavior, and lurks in the animal's saliva. Since most mammals are quadrupedal, they use their mouths and bite at anything that comes close to defend themselves. They do so under the confusion and pain induced by the virus, transferring their infected saliva when they do. A bipedal human, however, would use their hands to defend themselves when confused. Rabies is far from a zombie infection, as the organism is still conscious of their actions under the virus. Rabies symptoms also take multiple days to appear, so even if it was as dangerous as a zombie virus, we would be able to recognize and treat or quarantine the diseased individual before they were able to infect anyone else.
This takes us to the second reason we’re safe from a zombie outbreak: your immune system. Your immune system isn’t strong enough to defeat every pathogen you might encounter, but it excels at buying as much time as possible. It does this in a multitude of ways, with one of the most important being its means of slowing down the production of new viruses.
Every cell in your body contains a group of proteins called interferons, and when these proteins find part of a virus inside one of your cells, they attach to it and become activated, causing the cell to slow down its production of new DNA. Viruses rely on your cells’ replication of DNA to reproduce, and when that process is slowed, the production of new viruses is slowed as well. Essentially, interferons give your immune system a few hours to prepare before large numbers of viruses begin to appear in your body, and in our scenario, a few hours to quarantine or treat someone who’s been infected with a zombie virus. Even after a virus has multiplied and spread throughout your body, your immune system does everything in its power to make sure the virus has a terrible time there. Millions of immune cells wipe viruses out in mass with the help of side effects like a raised temperature or inflammation which make conditions for the virus absolutely miserable.
Your body also contains a group of specialized cells that kill cells who show even the slightest problems, called natural killer cells. These natural killer cells go from cell to cell and if one shows any sign that it might have a virus in it, the natural killer cell makes the infected cell undergo apoptosis, or cell suicide. Natural killer cells are also your greatest defense against when your own cells begin to harm you, as they kill any cell that appears to have something wrong with it, like in cancer, (or a zombie virus).
Your immune system obviously isn’t perfect however, as diseases that your immune system can’t beat make up a large portion of deaths around the world. Thankfully, we have one more reason not to worry about a zombie outbreak, and that’s the nature of pathogens themselves. All pathogens are subject to the forces of natural selection. If a pathogen can successfully reproduce and spread, it will live on. If it can’t, then the strain or species of pathogen disappears from existence. It’s simply not necessary for a pathogen to control an entire organism in order to reproduce, because viruses like influenza or coronavirus can spread effectively by using a few of a human’s cells instead of controlling the entire organism. It is also impossible to keep all of an organism’s muscles, cells, and organs functioning after that organism has died, even if it was possible to take total control of a living being. The rapid changes in the environment necessary to prompt these changes just aren’t present, at least not as of yet. For now, we’re left to watch for changes in pathogens as they undergo the gradual path of evolution, and to keep enjoying zombies in the movies, and movies alone.