I want a crack at this
Obviously, we wouldn't be talking about the good ol' Dawn of the Dead zombies, because they were living dead (if I'm not mistaken. I'm a slasher movie guy, not a zombie guy). I think that it would have to be some sort of neurological 'infection' (i.e. rage virus) that basically hijacked your body to do whatever it wanted to do. That would leave the question of how and why.
For the how, I don't think that it would have to be transferred solely through being bitten, simply because that would be too easy to stop from happening. Thick enough clothes, armor, or just being able to run quicker or defend yourself better would put a large enough damper on the infections ability to reproduce. I think that, while it could be transferred through bodily fluids, it would have to have a different and more effective way to spread. What is something that every human needs to live? Food, water, air. If the infection managed to be carried in one of those three alone, it could effectively spread throughout the human population.
For the why, that would depend on how the infection came to be. If it was engineered, then it would be whatever motive that the engineer wanted. If it was purely natural selection or selfish gene (and here's where I'm stepping into unfamiliar territory, so by all means correct me), would it simply want to be the "top of the food chain"? Again, I'm no biologist so I don't know if that would be accurate or not...
Also, if the infection was able to be transferred through food, water or air, it would most likely infect more than one person at a time, so I don't think that the baseball bat would work too well... I think that pretty much every zombie movie got the weapon of choice down: chainsaw