Herds?

[EvilNed]

However, don't forget the Russian Far East, for instance, where 7 million people live on a landmass roughly equivalent to that of the United States.

[krakenslayer]

The whole thing could be considered a herd. Depends how "close together" you think zombies have to be to be considered part of the same group. Think about how we have infested this planet. Taking the entire land area on Earth, and dividing it by the number of zombies, you get 104+ zombies per square mile. This means, even if they're spaced out evenly, no zombie is more than 500 feet from another zombie. That's not very far. Easily within sight of one another. Now consider that coverage over the entire planet. No place to run, nowhere to hide. They're everywhere, and close enough to one another. When viewed from high altitude, the whole planet is covered by one "herd". Excellent post and some very good points. However, one point I would bring up is that there are a lot of areas on Earth where the human population is far, far higher than 104 per sq. mile, and many places where it are a lot less than that - including parts of Canada, Africa, the USA, Antarctica, Greenland and various parts of Russia and Eurasia where there is no habitation for hundreds of square miles - mainly due to the inhospitable or remote environments there. So 104 per square mile is an average. Whether or not the zombies would diffuse out to cover the entire planet would depend on several factors including how resistant they are to extremes of temperature, how good they are at crossing rugged terrain, how far they can travel on foot before their legs drop off and their psychology - namely whether they are prone to wander aimlessly or whether they have a tendency to congregate in places they remember from life.

[Cinerary]

I prefer the word congregation over the word "herd".