Scott:

You'll have to come up with a dichotomous key for your monster classifications.

For Example:

1) Human-like monster is taller than 8 feet go to Giant Family
or
2) Human-like monster is shorter than 8 ft go to #3

3) Human-like monster has no heartbeat go to Undead Family
or
4) Human-like monster has regular heart beat go to #5

You get the idea. So, get started...it'll probably only take ya about five or six years to complete key of this type and its not like you're doing anything else right now. Hop to it!

Brian: Why are you such a sadist?

Yeah, it's good, not a bad effort....as far as it goes. But how about that thing from 'The Keep'? Non-corporeal until it's made flesh through the killing/suffering of others?

Don't get me started on the 'Killer Tomatoes' because I'm pretty sure they're NOT of Genus Homo (though the director might well have been).

And that's before we even start to consider animated monsters (and I don't mean like in Reanimator) such as those featured in Futurama (e.g. the Yarn people of Nylar 4, Brain Slugs and the Energy Beings) or Scoobie-Doo (I might be pushing it there, as most of them were, indeed, Homos).

Or are they all to be classified under the 'Hybrid' category? Actually, thinking about it, I guess you would....

Still, I would grade this paper A+ on the Bell curve for the following reasons:

1) It displayed a basic understanding of science
2) It made me snort with laughter and caused my kids to crowd round asking what was so funny
3) I have no desire to join the ranks of undead, Homo or otherwise

Now to translate from Twitterish to English...

First to correct your taxonomy: Bacteria are not protists. They actually get their own kingdom, predictably known as Bacteria. That out of the way, its very unlikely that the classic zombie, especially as we've seen in recent movies, would be caused by a bacteria of any sort. While its true that bacterial infections don't attack on the cellular level, they occur on the tissue level (which is why the level of infection is called the tissue burden). Bacterial infections cause damage by eating the tissues they inhabit, eventually causing organ failure and death.

While its not impossible that an unknown species of bacteria (or even better, an Archaea bacteria, a second kingdom of bacteria, for a total of 6) could release a toxin that induces zombie-like symptoms, given that known bacterial infections all basically work the same way, its not likely. Toxins are usually (like always) lethal or potentially so. Bodies left to bacteria will rapidly decay that even if they were zombified, they'd be in such bad shape after a day or two, they wouldn't even be able shamble around.

For example, M. avium complex (MAC) lives in freshwater. If you drink water, you've been exposed to MAC. For most people, MAC isn't a problem, because it grows so slowly, that our immune systems can easily kill it. But if someone with AIDS, a MAC infection will kill them in days. Literally 2 days. For a bug that takes me a week to grow in the lab.

So bacteria are out, which leaves viruses, right? Not really. There are two types of viruses (whose names I don't remember and don't feel like googling at the moment). The differences are the ways that they infect their hosts. All viruses work by injecting genetic material into a host cell, which causes the cell to make more virus particles until it lyses, releasing the new particles into the interstitial fluid. They can inject DNA or RNA (and reverse transcriptase to make viral DNA), but the big difference is whether or not they cause the virus factory to start right away or wait. Some viruses actually add their DNA to the host genome and it will lie dormant for an arbitrary number of generations until some condition is met that causes the factory to activate and standard cycle to resume.

Scary, right? Yes and no. Like the bacteria, we've evolved alongside this stuff since before cells had nuclei. Just like cats and simians, we'll eventually become naturally resistant to HIV, just like we're resistant to the common cold and the flu (ie, it sucks, but you generally don't die from it). Even having viral DNA bonded to genomes isn't really a problem, because viral particles and proteins aren't intrinsically harmful to us, and may have contributed to our evolution. New, scary infections tend to kill you so fast because of a lack of natural immunity, while common infections tend to bloom and get smacked down because of our racial familiarity. This effectively rules out traditional infection as a source of zombies.

Which leaves us with something most people haven't even heard of: prions. I do assume that you've heard of mad cow disease, also known as Bovine Spongiform Encephalopathy (BCE), called that because it eats holes in the brain, causing it to look like a sponge. What makes BSE such a problem is because its caused by prions, which we have to defense against. Unlike bacteria and viruses, which are invading foreign organisms (or your own cultures going nuts like PO'ed morlocks), prions are misfolded proteins. If you have a PS3, you might know that it comes with something called Folding@Home, which uses the Core processor to compute the extremely complex 3D shapes that proteins fold into. Proteins are a series of peptides that are generated by DNA sequences. These peptides merge into really long strings that fold up on themselves to form specific shapes. Humans have between 20,000-25,000 genes, which generate millions of proteins, each of which has a specific function that can only be accomplished if has the right shape.

So what's a prion, and what does all that have to do with them? Simple, prions are misfolded proteins. Unlike "normal" misformed proteins, which are harmlessly reabsorbed by the cell, prions are not only able to move about with impunity, if they come into contact with a properly formed protein of the same type, the prion will refold the normal protein into a new prion. Just think of prions as the Agent Smith of the cellular world (I just came up with that!).

Prions can be spread by any number of vectors, including eating (that's how mad cow gets spread) or direct contact with infected blood or saliva. While skin contact is probably not enough to cause an infection, contact with broken skin or a mucous membrane probably would be (like that poor bastard who got it in the eye in 28 Days Later). Prions can also cause selective damage to the brain, destroying personality, higher reason, most memory and skills, but leave the autonomic systems intact. Such an infection might cause damage to the skin and soft tissues, making identification of the infected relatively easy. Unfortunately, these unlucky people would still get hungry, but would lack the ability to remember that all of the walking meat around them are not supposed to be food. Such reckless eating habits would certainly cause secondary prion infections in the food, I mean people who survive the attacks. We still probably wouldn't see the lightning-fast infections in some movies, but considering how fast protein folding works, zombification would still be rapid (like on the order of tens of minutes to hours to days).

So there you have it! Prions are some really nasty scary stuff and we really don't know nearly enough about how they form, or how they can act as an infectious vector. Also keep in mind that most of this is from what I remember from cell biology classes, and is not my area of study. Check out http://twit.tv/fib27 for a discussion of Folding@Home and http://twit.tv/fib12 for a discussion of proteomics.

A geeky nitpick: species designations are not capitalized.

To dssstrkl
As someone who has enjoyed English beef during the Mad Cow outbreak, Prions are indeed very scary.

More scary is Necrotising fasciitis.

Scott, your Monster Taxonomy is really very educational, and I am going to buy your Field Guide as soon as it hits the Bookstores. It will be fun to go monster watching and to be able to identify what I am seeing. ( I have a feeling my next door neighbor is a little... "off".)
Having listened to, and read, your wonderfully creepy and wildly horrific novel, INFECTED, how do you classify the Triangles?

Dsssrtkl: What the hell, man, your comment is longer than my whole blog post. Are you trying to move in on my turf? Keep it up and I'll have to use prions as my insanity excuse at my trial ...

Thegreenman: So what you're saying is that during the height of Mad Cow, you were munching down on poorly cooked burgers? Forgive me if I don't invite you to my barbecue ...

acbook: The triangles from INFECTED are alien bio-technology, so they actually fall into the "killer robots" taxonomy under the special biotech heading. Genus and species? Triangulacis Shutthehellupicus.

The whole zombie classification would depend on what kind of zombie, depending on what exactly was animating it. If it's something manipulating the central nervous system (or regenerating and energizing it, as is implied with the Night of the Living Dead zombies and the returning Venus probe radiation and all), the destruction of that nervous system would make it indistinguishable from anything else in Cordata. (Yes, I've had a whole day to think about this since the twitter thing...)

Something magically animated, like the Templars from Tombs of the Blind Dead, would fit more what you have for zombies, as would, as you point out, 28 Days Later and the like.

But really, that just puts some of them in Undead, rather than their own category.

Tashkal: Ahhhh, I feel so at home among my fellow science-geek/monster-fans!

The major thing to remember in taxonomy is that things are arranged according to how they reproduce and how their genome relates to everything else. That means that at most, human zombies would be a different species than humans, but would remain in the Homo genus, since they presumably remain basically identical to uninfected humans. The physiological and behavioral changes associated with zombies are all phenotypic changes that have very little genetic causes.
As far as a taxonomist would care, the only difference between humans and zombies is that humans have sex to reproduce, while zombies bite people (or whatever). There would probably be a lot of debate about this, since that's not really reproduction in the classic sense; its spreading infection, not one's genome. Personally, I would think that zombies would still be considered humans, albeit humans who need to have their heads blown off and napalmed with extreme prejudice, but still Homo sapiens.
As far as alien taxonomy goes, I think you'd end up with a bunch of stupid names, as people like to name new species after their themselves or their wives. Genus and species can be fairly arbitrary, so the hatchlings from Infected would end up with some stupid name like Siglericus horriblis.