I am getting closer. I have some good ideas that give behavior similar to a shambler.
Toxoplasma already covered in this web site. Initial symptoms similar to the Flu. Then, is pretty much dormant in humans. Some recent work indicates it may affect the psychology of individuals. It has also been shown to mess up the behavior of rodents. Instead of smelling cat urine and running like hell, they are attracted to it. Most other fears are not affect. Such as fear of open places, fear of dogs, fear of strong strange smells.
african sleeping sickness. Mental capacities slow deteriorate making the person more and more like a vodoo zombie.
People's mental capacity deteriorates to the point that they are nearly statues. Interestingly, this was first described during the Kiss of the Spanish Lady influenza outbreak in World War 1. Additional cases have been reported since then, but never again reached epidemic levels.
I have not worked in the "reanimated dead" part yet. Still working on that. Some of the canon out there is just so wrong from a biology part I am having trouble reconciling it.