Rage VirusAppeared In: 28 Days Later (2002), 28 Weeks Later (2007) Characteristics: Increased strength and endurance, uncontrollable rage, and loss of higher brain function. Initially engineered out of a desire to pacify populations and reduce anger, scientists at Cambridge University accidentally caused the virus in question to mutate, and drive infected subjects to be overcome with murderous anger. The aptly-named Rage Virus is so potent, a single drop of infected blood or body fluid - usually transmitted through bites, vomiting, or arterial spray - can turn a subject within seconds. Blood vessels throughout the body rupture, and the rage takes over. Some dispute 'zombie' being applied to director Danny Boyle's Rage-infected, but given the symptoms (and the original film rejuvenating the zombie genre) we'll allow it.
The SwarmAppeared In: World War Z (2013) Characteristics: Insectoid swarm behavior, sprinting to spread infection. Like the original Max Brooks novel, the film never names the exact cause of the viral outbreak. And while the filmmakers refuse to use the word 'zombie,' the title makes the point clear: these are zombies, and on a scale never before seen on film. Director Marc Forster has taken some liberties with the source material, ramping up the speed and ingenuity of the infected; with bites causing infection within 12 seconds, the swarm mentality that soon develops when one infected is introduced to a crowd was evident from the first trailer. The brief glimpses of zombies pouring over defenses and through narrow streets like a human tsunami, and constructing towers of flesh like ants made the movie's zombies one of our favorite takes on the genre before even being released.
Smart, Still DeadlyAppeared In: The Crazies (2010), Land of the Dead (2005) Characteristics: Homicidal aggression, with some higher brain function and social skills intact. As terrifying as it is to imagine a pack of reanimated corpses slowly marching in a quest for flesh, there's something far more chilling about the same infection taking hold quietly - it's easy to know who to trust when the monster's flesh is rotting off its bones. Whether it's George A. Romero's Land of the Dead (2005), where the seemingly mindless zombies begin to form a new social heirarchy, or Romero's The Crazies (1973), granting everyday citizens of small town America the same unwitting need to kill, brains are what take zombies from chilling to downright terrifying. Some might argue that the ability to think, reason, or strategize is something that, by definition, a zombie could never possess, but films like these address the same unanswerable question: what happens to the brain when the body is zombified?
T-VirusAppeared In: Resident Evil (2002) and ensuing sequels Characteristics: Psychotic rage, loss of higher brain activity, and overriding need to fuel cellular energy through consumption of flesh. The product of the shadowy and morally ambiguous Umbrella Corporation, the Tyrant virus (or 'T-Virus') was designed to be the perfect biological weapon. But as the Resident Evil series of films has shown, it's a bit hard to contain once it gets out. After introduced to a subject - via air, water, or bodily fluid - the T-Virus takes over cells one by one, replicating itself along the way. Overriding cellular mitosis means only enough energy is produced to perform basic functions - walking, clawing and eating - so alive or dead, the infected human is reduced to a walking mouth within hours. We don't know where the drive to wrap one's head in canvas or seek out giant blades originates, but the T-Virus' ability to alter DNA other than humans - dogs and bees, for instance - makes it one of the more memorable (and enduring) varieties of zombie outbreak audiences have encountered.
Appeared In: The Night of the Living Dead (1968) and ensuing sequels. Characteristics: Increased endurance, loss of higher brain function, shambling gate and undying need to feed. It was director George A. Romero's idea of 'zombie' that both popularized the term in the horror genre, and laid out the basic rules. The mindless, shambling, reanimated corpses didn't reach mainstream success until Romero's follow-up Dawn of the Dead (1978), but have acted as the rubric ever since. Romero never explained what caused the zombie outbreak, but all it takes is a single bite to turn a breathing human into a zombie-in-waiting. The bite slowly kills the victim, who then reanimates after death. Endless supplies of energy and an overpowering need to feed are the hallmarks of Romero's zombie, even if the slow stalkers aren't too hard to avoid. Why did Romero's social commentary on consumerism, the military, and post-Vietnam era society have such a lasting impact? Well, considering that every person becomes a zombie after death - even natural - this kind of outbreak isn't one you survive; from the moment it strikes, everyone will eventually become the enemy.