This is where I detail my theories about the causes and origins of actual zombies.

Discuss here: Real-World Zombie Biology



* What constitutes a "zombie" in this discussion?

For the purposes of discussing an actual real-life occurrence of zombism, here are the criteria:

A human that desires to attack other humans with animal-like ferocity, total disregard for self preservation, lack of refined skills, and utter lack of morality or conscience.

*Why would they attack us and not each other?

It is in fact possible that certain kinds of zombies might attack each other. If the sole drive for their attacks is the desire of violence, then they would logically attack each other as well as those that are uninfected and animals.

However, there are two ways they could distinguish between zombie and human if enough of their base instincts remained to influence them to group with their own kind and act hostile to outsiders (the uninfected).

One is by smell. Humans and animals emit pheromones. Basically we smell a certain way. This is why hunters use scents and scent blockers. The zombies could potentially be able to smell us. Instinctivly, they know that "human smell - attack, eat, good nutrients, foe; zombie smell - no reason to attack, or eat, friend"

The second is by heat. Just like humans use different cones in their eyes depending on whether it is light or dark, perhaps the virus that causes zombism renders the undead able to only see limited parts of the spectrum that includes infrared. If the infection resulted in their body temperature being higher or lower than ours, they would be able to make that distinction to tell friend from foe.

*How fast does the infection spread?

It really depends on the type of pathogen that causes zombism.

Theoretically it would never be "Instant" but it could be rapid (several hours) to delayed (several days).

If a zombie outbreak does happen it would most likely be a type that takes at least 12 hours because this delay would allow the infection to spread. If it was instant, it would not really spread far.

Examples:

A scientist experimenting with perhaps a "miracle cure" pricks himself with a needle accidentally and is infected. If the infection is instant then he turns right then and there. Maybe manages to bite one or two other scientists. Security is called, maybe one or two more people get bit, but the infection goes nowhere. The infected are contained, etc.

A scientist experimenting with perhaps a "miracle cure" pricks himself with a needle accidentally and is infected. The infection is delayed by several hours. The scientist gets a blood test taken "just in case" and then takes the rest of the day off. He goes home, turns, and bites his family (wife, son, and daughter). He is shot by police, and the family is taken to the hospital. They turn there, and bite everyone in the ER. And on and on. Maybe the whole town ends up infected and the military have to be called in, but eventually the outbreak is contained.

A scientist experimenting with perhaps a "miracle cure" pricks himself with a needle accidentally and is infected. The infection is delayed by 12 hours. The scientist gets a blood test taken "just in case" and then takes the rest of the day off. He wakes up in the morning feeling not so good, so heads into the lab to see the results of his blood test. On the way there he turns and bites a guy hailing a cab on the hand. The guy manages to escape into the cab, and goes to the airport. He is headed on a business trip to Japan. You can see how much more this is spreading.

* Fast zombies or slow zombies?

Both. They start out fast and somewhat coordinated but as their bodies wear down they get slow.