Zombies!

Zombies have become a pretty big thing lately, in case you hadn't noticed. They take a variety of forms, but they all share three key characteristics: they're strong, dumb, and highly infectious. They're usually also very slow, and I'm going to assume slow zombies for most of this post. The problem, then, is how zombie outbreaks get started in the first place. Oh, sure, it's pretty obvious why zombies are dangerous in large numbers. They swarm in from all sides, and someone gets bitten at some point. But how does one zombie (or even a few) start the kind of world-destroying outbreak depicted in movies, books, and video games?

See, most zombie viruses have a lot in common with rabies. They make the infected stronger and more aggressive, and they spread by bite or blood contact. Now, rabid creatures are dangerous, but they die after awhile. Zombies usually don't. But rabid creatures are also quite fast, in stark contrast to zombies. Given that zombies aren't cunning enough to be successful at hunting without speed or numbers (and they need to hunt to get numbers), I'd judge that slow zombies are a fair bit less dangerous than rabid creatures. And yet rabies has yet to wipe out any major countries. So in order for a zombie outbreak to be successful, it'll have to use some other infection vector.

One possibility is mosquitoes. If the zombie virus is blood born, mosquitoes might easily spread it. This would be particularly terrifying, as mosquitoes are a lot harder to avoid entirely than zombies. Now, mosquitoes don't feed off of corpses, so the disease would need at least a few day's incubation period. What's more, an area that's been fully infected for a couple weeks is probably fairly safe, as all the mosquitoes will either leave or die out from lack of food.

Another option is that the virus isn't a virus at all, but a fungus. Similar to this fungus, it could control humans and turn them into zombie-like automatons. In this case, a bite is unlikely to be infectious, but zombies may release spores. As with the mosquitoes, this infection vector would be very difficult to avoid, and even killing the zombie might only release more spores. This also opens up some interesting aesthetic options beyond "burnt-out shell of a town." An infected area could be overgrown with strange fungi, creating a truly alien atmosphere.

The third (and possibly strangest) option is the zombie virus as an STD. Now, for this to work, it has to have a long incubation period, probably at least a month. It should also probably be infectious through the traditional bite-vector, if only so that the zombies still pose a credible physical threat to survivors. Still, the sheer infectiousness of STDs combined with turning people into zombies could be a terrifying thing indeed. Imagine a hybrid of AIDS and rabies.

Next up, the "ghoul" option. Imagine that the first person to be infected is a superior breed, call it a ghoul, and that sometimes other infected become ghouls instead of zombies. Ghouls need only be faster and more cunning, and you make the traditional rabies-vector dangerous. After all, now you have a creature that can actually hunt. One can actually imagine a lone ghoul picking off victims until the horde reached critical mass. This gets all the more dangerous if the ghoul has any kind of control over the lesser zombies, because then the horde gets a general. Scary stuff.

Finally, we have fast zombies. The difference between fast zombies is not in their infection vector, but in their physical threat. Because they would actually be capable of human-like speed, fast zombies would be much harder to avoid, especially for unarmed civilians. They would probably also have limitless stamina, being zombies, so eventually they could run down practically anyone. Basically, go back to the rabies analogy, but now the rabid creature is tireless and doesn't die on its own. That might just be enough to tip the scales and start a true outbreak.