How long would a zombie outbreak last? By that, I mean, how long would the zombies be around?
Absent some supernatural explanation for a zombie outbreak, zombies (whether the "traditional" dead, but infected human; or the alive, "rage" type virus) must intake and use fuel or energy (i.e., food) and transport that to the body tissues in order to move and function. How they do this may have a significant impact on how long zombies are around.
If we look at the "rage" virus zombie, we have a human that is not dead, technically, but overcome with a virus that drives them insane, and perhaps makes some other physical changes or mutations (bigger, stronger, faster, for instance). But, in the end, you still have a living creature that needs food. In the initial stage of an outbreak, the "food" is plentiful, so lots of zombies moving around. But eventually, the "food" is gone. The "rage" type zombies are going to starve just like any other creature. They will rapidly decline and disappear. Will they mostly be gone with 28 days, just to pull a figure from the air? Don't know, but they would certainly be mostly dead from starvation within a few months.
Turning to the "traditional" zombie--the infected dead--how the zombie consumes and transports energy to the areas of the body is important because it places different constraints on the system. Because the traditional zombie can only be killed by destroying the brain (rather than the heart or massive wounds that would kill a human) is clear that the zombie virus is a hive-type creature that uses the human neural network (i.e., brain) to control or animate the body, but doesn't not make use of the heart and circulatory system. So how does it get energy to the muscle tissue to move the body around?
One possibility is that it doesn't. Biting and munching on people is a means of spreading the virus, but does not actually feed the zombie because the food is not digested and distributed to the tissue. In this case, the energy would necessarily have to come from the muscle tissue itself or surrounding tissue. In this case, the zombie body is actually cannibalizing itself over time in order to move. As the zombie moves, the amount of tissue will decrease--the zombie will take on an emaciated and, finally, almost skeletal appearance, slowing all the while because it has less and less energy reserves. In order to conserve energy, the zombie will need to keep from moving. This actually matches with the idea of the zombie being an ambush predator. It will go into a hibernation state until motivated to move and attack. However, this only delays the inevitable--eventually, the zombie starves. A zombie outbreak like this would be like a match flaring--very large aggressive outbreak for a very short period of time, but with the zombie starving to death in just a few weeks, if even that long.
Another possibility is that the zombie virus actually grows a different circulatory system within the body. Essentially, shifting from merely a virus, to an actual parasite within the human host. This type of zombie would be able to digest food, but is perhaps more vulnerable to massive body damage than the virus-only zombie. However, the advantage is short lived. Deprived of its food stock, the zombie will still starve over time.
Is there any historical support to zombie outbreaks spreading fairly quickly and then dying out. One possibility is the collapse of the Minoan and Hittite civilizations. Although
the eruption of the volcano at Thera probably contributed to the decline of these great civilizations, it is also apparent that these civilizations ended quite abruptly, with evidence of burned cities and sudden collapse of the population. Certainly a hallmark of a zombie apocalypse.
So, to sum up, food constraints will decimate the zombie horde quite quickly--perhaps in as little as few weeks, but certainly within a year, the zombies will have disappeared.
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