Nobody understands how those who died can rise again, let alone be driven by a diapolical appetite for human flesh. Regardless of the science behind it, zombies are now a part of everyday life; perhaps even the most important part. Many people call them God's punishment made manifest, while others think it was a governmental scheme gone wrong. Regardless of what they are labeled as, they are not only the reason why the world is in the state it is now, but their very exsitence is why humanity will take many, many years to recover, if ever.
What do you call these things again?
Once the first patient turned, complete chaos ensued. With it, communication lines quickly failed, leaving the whole country (and world) disconnected from each other. This led different people in different areas around the country to give the Dead different names. Of course, helped by popular media of the time, the most widely used name is 'Zombies'. But this wasn't the only name; these names include:
Zombies, Infected, Dead, Biters, Lickers, Demons, Cold Bodies, Creepers, Floaters, Geeks, Lamebrains, Lurkers, Monsters, Roamers, Rotters, Skin Eaters, Empties, Walkers, Wanderers, Runners, Rippers, Deadheads, Growlers, Stinkers, Hissers, Puppets, Ghouls, Revenants, Zacks, Zeds, Fleshies, Bonies, Hungries
The names given to those creatures depend on where the people live; maybe the climate there plays a role? Or what these did for a living. The names used by your character can be used to describe their cultural and social backgrounds. Get creative.
It is unknown how exactly the first zombie came to be. If it was known, a cure would have been found already by reversing this effect. But by this point, this is not the question that keeps people awake at night. It is the following one that people think most about: Why do they eat humans? To answer this question, one must understand that zombies are basically human rotting bodies. To keep them together, they need to supply spare elements taken from a human body. Eating a human body is the most direct way to get those, considering how inefficient is their digestive system. Since they cannot fully digest food and use those elements back to synthesize bodily components, accessing human intermediates is the best compromise.
Considering zombies are dead humans with decomposing bodies, one might think that a zombie would be much weaker than the average human. Sometimes a zombie will be stronger than a normal person, somewhere around twice strong based on extreme stress hysterical strength response. Every time a human pushes a muscle past about 70% of its normal, non-hysterical maximum he gets micro-tears, these tears take several days to heal under ideal conditions and the more tears the longer healing takes, continuously generating tears will wear the muscles away to nothing within days. However the zombies are going all out, their brains not limiting their muscle output to protect their bodies. This leads many 'old' zombies to be uncapable of doing basic things, having their muscles torn by hysterical, unchecked activity.
They require none of their internal organs with the exception of one, the brain. Destruction of the brain is the only sure way to kill a zombie. Freezing or burning a zombie can potentially kill it. However, this is entirely based upon the amount of damage their brain sustains, and is an unreliable method of extermination. Decapitation will render the body nonfunctional, but the head will remain alive and still pose a threat.
To say that all zombies are the same, to say that all are just slow, mindless, rotting corpses would however be wrong. The virus that started it all apparently inserts its genomic material into the host cell genome and can sometimes cause mutations. Thankfully these are very rare and most people don't know of mutated zombies anyways. These mutations vary widely, but are all equally dangerous. One thing to note however that most of these 'mutated' zombies posses some kind of cunning and semi-intelligence, making them much more dangerous than he usual zombie and also capable of harnassing the power of their mutations. These are some of the documented types of said mutations:
Runners: The most commonly found type of mutations. Due to an unknown genetic mutation, the leg muscles of these zombies don't tear easily, even after running supremely long distances. They are fast, much faster than the average (and above average) human and unlike humans, they don't stop until they get to their target.
Jumpers: Very similar to the Runners (although not as common), the muscles of these zombies' legs don't tear. However unlike thr Runners, their leg muscles grow to twice its normal size. This of course prevents them from running quickly due to the big mass around their legs, however it enables them to jump very large distances in the air. Settlements that encountered this type of zombies have mostly fallen due to the fact that this type of zombies can easily jump over the settlement's walls and infect residents inside.
Bloaters: Their mutation has caused them to produce immense quantities of bile inside their bloated bodies. Upon dying by massive blunt force trauma to the body, their bodies explode, sending everyone close to it to their demise and covering the rest with its disgusting bile.
Screechers: Perhaps the most dangerous ones are the so-called Screechers. When feel threatened by a human or a group of humans, this zombie lets out an unholily loud scream at around 200 decibels. These sounds are enough to make a healthy human feel like his eardrums will burst, immobilizing him temporarily. This allows the zombie or even others to feast upon the immobilized human's flesh while he doesn't have a chance to fight.
Sniffers: This type of zombies, also called 'Dog-Zombies' or 'Wolf-Zombies', are not very common. These zombies are able to detect humans around them by their smell. This combined with their cunning makes them very dangerous zombies everyone hopes they would never run into.
Hulks: The rarest ones so far are the so-called Hulks. They are the brute force and the merciless savagery of zombies manifest. They are bigger and much stronger than your average zombie, being able to take about as twice the damage needed to take down a normal zombie. They have been mutated that their flesh and muscles decay over much, much longer time periods, making them almost as fresh as a human, but with the brute force of an apex predator. Almost nobody lived through an encounter with one and lived to tell the tale.