The glut of zombie-related media in the last few decades, especially the last five years or so, has identified several distinct varieties of zombie. While the differences between them can be extreme, most zombies have several common features.

The most important trait of the zombie, across all types, is their need to feed. Every zombie is driven by only one ambition - eating. While some zombies seem to prefer brains, this has become less important in recent years, and the desire for living flesh in general has emerged as the standard. Some zombies ignore animals, while others have no problem chowing down on them should they get the opportunity. The appetite of the zombie is unrivaled - they eat even after they’ve just had a meal, and some types of zombie will seek out food even if they have lost any discernible digestive system due to trauma or amputation (such as the ubiquitous crawling zombie).

The Supernatural Zombie



The major distinction between zombies results in three types - the supernatural, the biological, and the psycho. Supernatural zombies often have no scientifically proven reason for their existence. They may have risen up due to a religious prophecy, such as the famous Romero line: “When hell has no more room, the dead will walk the Earth.” They may be corpses reanimated by a magical spell. Their origin may be unclear, but they are able to sustain great bodily harm and continue to “live”. Often, the only way to dispatch them with any finality is through partial or complete destruction of the brain. In contrast, biological zombies are infected by a bacterial, viral, or parasitic agent. They are living human beings, but they are ill and display extreme aggression, violence, and physical abilities, similar to rabies among animals. These zombies may not be able to withstand as much force as a supernatural zombie, and can often die without serious brain trauma, as their nervous and circulatory systems function much the same as they did before infection.

Supernatural zombies can still be referred to as “infected”, even though the exact mechanism of the disease may be unknown. In this sense, it is possible that being bitten is more akin to a curse than a biological process, similar to the bite of another legendary undead creature, the vampire. Slow, shambling zombies that seem to be supernatural in nature are often referred to as “Romero zombies” after the filmmaker who established a popular series of zombie movies in the 1960s, George A. Romero. Romero zombies cannot run, and at best have a lurching gait. Generally a quick walking pace is enough to outdistance them. However, because these zombies cannot become fatigued, they can catch up to a tired living person with enough time. They also have a tendency to loiter in places where they can easily surprise the living, such as darkened stairwells or basements, although they certainly do not have the capability of hiding in the true sense. Despite their base mental abilities, they can often be found in groups, which can be extremely dangerous.



The Biological Zombie



The other type of zombie is broadly referred to as the “O’bannon zombie” or “fast zombie” after filmmaker Dan O’Bannon, who directed the Return of the Living Dead series. These zombies are usually biological in nature, and are capable of moving very quickly. They often have more strength and aggression than a supernatural zombie and can quickly rip through some barricades and barriers. They may or may not require brain trauma to be stopped. Some of them, like those in 28 Days Later, are actual living humans and can die of exposure or dehydration.



The Psycho Zombie



These creatures are not zombies in the traditional sense, but they exhibit several features that make them quite similar. In general, they have been exposed to some kind of external stimuli that has made them extremely aggressive, violent, and disinterested in communication. However, some of them may be able to speak and even reason to some degree, as in the film The Signal. They are usually not interested in actually eating people; they seem to only want to murder them. They are still human, but have no real quality of life and may not be able to care for themselves properly.



Infection



Zombies are all contagious in one form or another, which only serves to enhance their dread. In addition to being able to kill humans with brute force and sheer numbers, some can also convert a healthy human to a zombie. This is a terrifying prospect, and many people faced with infection opt to take their own life to prevent the conversion. Some zombies can infect others with a bite or even a scratch. If there are no other serious injuries, the victim can usually expect to convert within a few days - less if there are multiple bites or if they are already suffering from grievous wounds. In other scenarios, any human will reanimate after their death, regardless of the nature of their death, even if they never came into contact with a zombie. This is probably the bleakest of scenarios, as it gives zombies a distinct advantage in their recruitment.

In a biological zombie scenario, the virus or bacteria may be spreadable by contact or inhalation, and some people may be immune to it. In any zombie apocalypse, the zombies themselves are merely the tip of the iceberg. Disease, malnutrition, dehydration, accidents, and violence amongst the living are also serious concerns, and the zombies get a free teammate if someone dies through any of those means. Another less common means of transmission is through a Typhoid Mary-like carrier, such as in the video game series Left 4 Dead, where the characters are symptom-free but spread the disease to any uninfected they come across.

Psycho zombies are usually not capable of infecting others directly. More of them may be created through unorthodox methods, such as being exposed to an audio signal or television broadcast (see the book Cell, for example).

Regardless of the means of infection, or the nature of the subsequent degeneration, there is generally no cure, vaccine, or treatment for the zombie state. Survivors must be extremely conscientious of their surrounding environment at all times - even one bite could mean death and a quick conversion to the zombie state. For some zombies, the only hope for a bitten survivor is an immediate amputation of the wounded limb or surrounding flesh. Even this method is not guaranteed to prevent a conversion, should the survivor actually live through the subsequent shock and blood loss.