I need a form of transmission for a zombie-like disease that would allow it to actually infect the world. Most people who work with the realism of the zombie infection agree that biting alone would not work. Remember that time when your dog got rabies and every other dog got it? me neither. So to me this raises the question, what form of transmission would realistically be the most effective for a fungal zombie disease?
Make it airborne, like a flu. This might make it a bit too infectious for your average zombie story. And it removes a lot of the excitement of survivors avoiding infection if they just get it because a zombie sneezed.
Or make it water borne, like cholera. In developed countries, this allows it to be transmitted by food preparation. In developing countries, it can be by poor sanitation. This makes it very infectious while still allowing survivors to avoid infection through their own actions (good sanitation, boiling water, canned food). And since it's in the digestive tract, biting still works.
Most importantly, increase the time when the infected are showing no symptoms but are infectious. Instead of the usual minutes or hours, make it days. This gives people ample time to spread the infection before they, or anyone else, realizes there's a danger.
They are released in the air and you can get contaminated just by breathing them.
Cordyceps are an example of this. If you have heard about the game The Last of Us, there is an infestation of zombies propagating the infection by releasing spores.
Yet, people could counter that by wearing masks.
What if the zombie disease is spread as part of a standard compulsory childhood vaccination? Like for instance, as part of a MMR vaccine? And it lies dormant til puberty or early adulthood, or some environmental stimulus like a solar flare or whatnot. An entire country of people could be vaccinated as infants, and turn into zombies 20 years later.
You might want to check out John Ringo's 'Black Tide Rising' series. He postulates a bio-engineered virus that has two different infection methods; the first is a flu-like "sneeze and infect" carried by pre-symptomatic people, followed by a zombie-bite blood transmitted infection. If you postulate that the zombie cause was created (bio-engineered), you can (with sufficient hand waving) add as many different infection paths as you like, no matter how complex the virus becomes or how unlikely the virus would have naturally evolved. The problem at that point would be thinking of a way to avoid 100% human fatality.
While Schwern's idea of cholera is probably the most effective one, I'd like to offer you a counter-proposal, a bit more consistent with people's general idea of zombies: zombie fleas.
The idea is simple: when a flea bites a zombie, it becomes a zombie itself, and carries on the disease. What's worse, just like other zombies they become immune to any chemical repellents, environment conditions and lack of food (since that are already dead). The only way to kill a zombie flea it to squash it, and squash it very hard. If you don't squash it hard enough, it will be immobilized, but it will still lurk somewhere, ready to bite. Alternative solution, is, as always, man's best friend: a flamethrower.
Those creatures should be able to make any mammal either infected or a carrier, so if one zombie flea bites a dog, soon enough all its fleas would become zombies as well.
The advantages of this approach are:
- You will address an issue often neglected by authors of zombie books and films (what exactly happened to fleas in Resident Evil? Are the magically immune?)
- It will add some drama (some character had a flea, but there is no way to tell whether he was infected or not)
- It will give your characters a reasonable way to avoid contamination: just don't get infested with fleas.