If there is one thing that can be said with any degree of certainty about the fictional zombie virus, it's that it generally takes over its host an incredibly fast pace. In both 28 Days Later and 28 Weeks Later, the "Rage Virus" has control of the victim in a matter of just a couple of seconds. In the movie version of Max Brooks' World War Z, it's generally a case of around twelve seconds, but can extend to a few hours. And that's very much the same in The Walking Dead. When the character of Shane Walsh (played by actor Jon Bernthal) is shot and killed, he turns in mere seconds. When Amy (actress Emma Bell) is fatally bitten during a "Walker" attack., the severe blood loss she experiences kills her very quickly. Her sister, Andrea (Laurie Holden), cradles Amy's body not just for hours, but overnight, before she transforms into one of the living dead. So much for the domain of fantasy, but how quickly do real viruses spread? The good news is, nowhere near as quickly as their on-screen counterparts.
Without a doubt the closest real world equivalent of the zombie virus is the rabies virus, since it is so often spread by a bite, and by the transmission of the virus in the saliva of the infected. The symptoms of rabies, to a degree at least, are somewhat zombie like: violent and agitated behavior, delirium, and a significant change in behavior, to the point where the infected often have to be isolated and restrained. But, even the rabies virus fails to act in the near-instantaneous fashion that the zombie virus so often does. The first symptoms of rabies are flu-like in nature, but - in stark contrast to the average zombie virus - do not manifest until, at a minimum, fourteen days after infection, and even as long as three months. There is a zombie parallel, however: when the physical and psychological symptoms set in and become apparent, there is very little chance at all of the victims surviving. As the Centers for Disease Control (CDC) starkly notes: "To date less than ten documented cases of human survival from clinical rabies have been reported."
Then there is the matter of one of the most feared viruses of all: Ebola, or, to give it its correct title, Ebola Hemorrhagic Fever (EHF) - first identified in 1976, in the Democratic Republic of the Congo, Africa - is particularly terrifying since, as the CDC admits: "Because the normal reservoir of the virus is unknown, the manner in which the virus first appears in a human at the start of an outbreak has not been determined." It is hypothesized, however, that when an outbreak does start, patient-zero has likely been infected by an animal, usually via a bite. As for how EHF spreads after the initial infection, it is generally exposure to blood, bodily fluids, or secretions of the person infected. Since one of the side effects of EHF is severe diarrhea, inadequate protection while cleaning the patient has led to infection of the care giver on a significant number of occasions. For all of EHF's ability to infect its victim, it's most certainly not a fast-acting virus at all. Its incubation period is anywhere from forty-eight hours to three weeks. Bird Flu - officially termed Avian Flu - is another cause for alarm in many people. Avian influenza primarily affects birds, hence its name, and can jump from species to species. The virus is spread by exposure to infected saliva and feces. In birds, it's an infection that typically kills within two days and with a nearly one hundred percent success rate.
What all of this tells us is that in the real world, viruses do not act in the way that they do in movies, television shows, novels and comic books - ever. Even in the event that a real-life zombie outbreak of The Walking Dead proportions really does occur, it is highly unlikely that anyone will suddenly reanimate and savagely attack the living in mere seconds, or even in minutes or hours. This gives us a significant advantage when it comes to combating a potential apocalypse: the infected can easily be placed into isolation as soon as they are bitten, without fear of them immediately changing into marauding monsters. Or so we can hope.