Quote from walker90234Now, there are also reported cases of people in the world who don't feel pain. Now, I don't know exactly what causes this, but I'm pretty sure that its some sort of damage to the brain, or nerves or CNS ect. Now, there do exist viruses and bacteria which, when they infect someone, "target" (i use that term loosely) specific parts of the body. Now, we can imagine a virus which attacks the same part of the body which, if it defects, will preent us from feeling pain.
Doing a bit of research it appears there isn't actually a "pain center" of the brain, the areas which process it are responsible for a ton of other things.
That and an infection in the brain would cause swelling, it's more likely that the pain receptors would become overexcited and cause more pain rather than reducing the body's ability to feel it. Plus there's a reason pain exists -- without it, you're much more likely to accidentally harm or even kill yourself before you realize it. It'd be disadvantageous, especially in someone with uncontrollable rage.
I mean it really is simple to say an infectious disease might exist which eliminates and exaggerates certain aspects of brain function in a way that perfectly mimics what a zombie does. But, it's really a cop-out and the amount of secondary brain processes which would be eliminated as well would probably render the person a vegetable.