If the zombie virus was airborne or waterborne we'd know all about it by now, we can safely discount that for the time being. Zombies are freaks, but still animals. They can recognise their own kind the same way we can, not to mention the smell of a human that only bathes in other humans and the sight of the bastards.
On the other hand, thermite rounds coated in phosphorous would kill and cook them in minutes. Mmm...