Zombies...what the what?!?

Zombie Herd

 will33t4food. (2011, October 17). the walking dead recapping: what lies ahead. Retrieved April 26, 2013, from TV Dinner & a Movie: http://tvdinnerandamovie.com/2011/10/17/the-walking-dead-recapping-what-lies-ahead/

There are some questions I have that I don't know the answers to, my intention is to post them on here and see if any of you have thoughts as to clear up my confusion.

1. I have to be honest when I say that I don't quite understand zombie herds. I mean if only the brain stem is activated then what causes them to follow each other? I get that they follow noise and living things, but what causes them to casually walk amongst each other across landscapes. For example in Season 2 Episode 13 Beside the Dying Fire they show the beginning of the heard that attacked the farm starting in Atlanta, Georgia where they followed a helicopter and just kept walking adding to their number as they went. (Darabont, 2010-2013) What I don't understand is what causes the others to join? What compels them to aimlessly wander together instead of stopping until they see something alive? 
2. Was the herd that passed them on the freeway the same herd that later attacked the farm? (Darabont, 2010-2013)
3. The zombies only seem to be intelligent in Season 1 via turning door knobs, looking in peep holes, climbing fences, opening doors, etc. (Darabont, 2010-2013) Why is this? What causes those zombies to be more intelligent then those that come in later Episodes? 
4. When birds or animals eat the zombies after they have been killed do they become zombies? Or does the infection only infect humans? Are there zombie pets? If so does it affect them the same way it does humans?
5. If the sensory motor area of the brain doesn't come back and there is no blood flowing through the zombies bodies, how are they walking around? Wouldn't the muscle tone erode and they wouldn't be able to stand let alone attack, especially after 3 years?
6. How is it that the zombies don't decay faster, it is Georgia and it get's pretty warm there, wouldn't the human body, if not alive, decay quickly?
7. How did the zombie infection initially start? Was it through vaccinations?  Through food? Water? What was the initial cause of the plague? And how did everyone obtain it?
8. Why does a scratch or bite activate the infection in the body's system? What triggers the activation in the system? Is it just a blood-borne infection?
9. Since the CDC was studying the zombie infection, was it developed as chemical warfare but went wrong in development and released onto the entire population?
10. When someone dies of old age does the virus still activate? If so, wouldn't there then be concern over older folks passing in their sleep and everyone waking to a walker?
11. Has burning or burying the walkers tainted the ground? What about the water? 
12. If a women who is pregnant becomes a zombie does the baby inside her become a zombie too and eat her from the inside out...kinda gruesome to think about. 

If you have insights or thoughts, please feel free to post. I will add to this as I learn and understand more and come up with more questions about zombies.

Works Cited 

Frank Darabont. (2010-2013). Days Gone By Season 1 Episode 1 [AMC's The Walking Dead]. Gale Anne Hurd.  Atlanta, GA: Stargate Studios.

Frank Darabont. (2010-2013). Beside the Dying Fire Season 2 Episode 13 [AMC's The Walking Dead]. Gale Anne Hurd.  Atlanta, GA: Stargate Studios.