To those whom think zombies are NOT possible:
It could happen one of two ways. Either a virus type bio-weapon, or the cure to a disease.Lets pick a deadly disease, Aids. Now lets say scientist create a "cure". Over time a medication or virus could cause swelling of the medula omblangota, thus causing increased aggression. The swelling of the brain causes some sort of neurological disorder shutting down parts of the brain. The person is now living only on anger and adrenaline. Now our "patient" is aggressive and mentally challenged. Unable to walk, talk, or function properly. Since adrenaline can keep the body going awhile after death the only way to stop the person would be destroy the brain or cause the person to bleed out.
Thats still not a zombie though. Zombies hunger for flesh. Many medications already induce odd cravings for dirt, chaulk, and soap. With an enlaged medula omblangota the subject would act much like a natural predator in the wild. Such as an alligator does. A craving for flesh would cause the zombie to simply attack anything that smelled like flesh.
The final zombie like attribute is spreading through a bite. The chemical causing zombie like symptoms would simply transfer through the saliva into the victims bloodstream and attack the brain. The major difference between hollywood and my theory being it would take days or weeks to develop. In wich case this virus, or chemical would be transmitable in the same manners as HIV. Since you wouldn't know you were a zombie until your brain swelled enough to trigger the reaction.