I feel an uncanny sense of uncomfortableness in what I'm about to do. Analogous to a parent preparing to tell their child there is no Santa Claus. But I believe it is necessary that everyone know that the theoretical "Zombie" or "Undead" Scenario is completely impossible. Since L4D is actually about an infected rather than zombies, the events that occur are indeed possible, while still highly unlikely.
The typical or "George A Romero" zombie scenario calls for a person who has died or is already dead to come back to life as a manic, soulless, flesh hungry/blood thirsty shell of a human that is slow-moving but resilient to the point that only cranial penetration (preferably with a bullet) can subdue it. Further, said zombie can thus spread this sad excuse of an existance to a normal healthy human through bites, scratches, and other blood-to-blood contact. Thus a person who is bitten by a Zombie, falls faintly ill, dies, and then immediately wakes up as a fangless daywalking vampire of human tissue. Thus, story plots can be built up from there and a long series of movies and remakes can be created.
But is a Zombie possible? Matter of factly, No. It is a scientific and medical impossibility for a Zombie to exist because human tissue needs life. While that is a fairly general statement, the truth of the matter is that the finer mechanics involved in every single organ, process, and cell in an organism are finely tuned so that they only work correctly when someone is alive and well. Once you distabilize any part of an organism, that same organism will have to compensate for it or else it will die. Death only occurs when the organism has been distabilized to a point that the body cannot recover and therefore cannot continue to function. Visually speaking, the correct correlation is: Stability --> Recovery --> Functionality. For a Zombie scenario to occur, an organism who has been distabilized to a point that the body cannot recover is still able to function after death. This goes against the above correlation and also goes against many other principles in medicine, biology, physics, and chemistry. To prove this, I'm going to have to get technical by taking you under the microscope and do a step by step process of how and why a Zombie is impossible, medically and scientifically.
The Zombie condition is technically a disease as it is communicable in all fictional occurances. However, this disease can also be contracted by someone who is already dead (explains why zombies can climb out of gravesites). For all intents and purposes, only a bitten, healthy human will be discussed here. Human X is bitten in the arm by a Zombie between the shoulder and elbow. During contact, a few thousand units (viruses or bacteria) enter the circulatory system, assimilate cells at an alarming rate, and start reproducing via apoptosis or cell death. Human X immediately begins to feel faint, and then as conditions worsen, more symptoms begin to be exhibited. Virus or bacteria has now spread outside the circulatory system and is now infiltrating nervous, immune, digestive, endocrine, cardiac, and pulmonary systems. As cells continue to die, organs begin to fail. Cardiac arrest occurs, oxygen is no longer distributed amongst tissue, brain activity ceases. Human X is dead. Now's where the impossible begins. Zombie pathogen must fully restart the heart, then partially restart the brain, lungs, liver, and kidneys. Afterwards, the pathogen must unclog the circulatory system which has most likely clotted due to lack of cardiac activity. Let's take each of these one by one. The pathogen "re-activates" the sinoatrial node of the heart to begin pumping. The SA node uses salt ion gradients to create action potentials or electrical impulses. So, the pathogen must be able to generate one of these impulses at such a voltage that anyone touching the victim would likely feel some sort of shock. Also, the typical heart that has stopped beating will begin a heart beat with systolic and diastolic phases out of rhythm so the pathogen must also be able to sync these back up. Around the same time, the pathogen must also infiltrate the brain and spinal cord and begin basic neural activity with little to no circulatory activity. Keep in mind that this pathogen must also be able to perform all of these activities in an anaerobic environment. Next, the pathogen must kickstart diaphram contraction for oxygen exchange, or be able to provide the circulatory system with some sort of gas exchange to remove toxic gasses from cellular processes (since typical Zombies don't seem to breathe). Muscles need continued stimulation from potassium and calcium ion chains for ATP-ADP activity which the pathogen will also need to provide. At this point, muscles can begin new contractions, eyeslid open, eyes and visual cortexes have already started to degrade due to necrosis and have glassed over. Zombie is now able to move about with limited function.
If you were having trouble following my step by step process, all you need to know is that the steps a pathogen must take to restart a dead human being are far beyond the capabilities of a pathogen. We're talking about revival of a larger and more complex organism using smaller and simpler organisms. Not only is a pathogen not-equipped to do such a task, but pathogens tend to destruct and only destruct and never resurrect. To do so implies intelligence which is also impossible because pathogen typically rely on the host for all functions and processes. To make a long story short, the Zombie condition is a bunch of bullshit. It is a myth that is practically insulting to anyone in the scientific and medical fields.