A mutated strain in a pandemic is a likely start. Going unchecked in a 3rd world nation and incubating into and unmanagable strain.
"When the Lamb opened the fourth seal, I heard the voice of the fourth living creature say, "Come and see!" I looked and there before me was a pale horse! Its rider was named Death, and Hell was following close behind him. They were given power over a fourth of the earth to kill by sword, famine and plague, and by the wild beasts of the earth."
Prime refresh my memory, they only named of one of the four horsmens horses. I believe it was Death's horse. Wasn't the hourse called Pestilence?
HIV has now been in the US since the early 1980s. Back then the CDC was able to trace the disease back to the "Typhoid Mary", the person that introduced it to the US population. It has been here for a couple decades now and although it has been somewhat controlled it is still a problem among the population. Influenza, the flu, has decimated communities. It has been around for centuries. We can control it but how long until it gets out of control? Rabies is a viral infection/disease that has been around for centuries. Uneducated people thought it to be lycanthropy. Irritability, headache, pain, itching or a twitching at the infection site, and fever. As the disease progresses, muscle spasms in the throat and respiratory tract affect breathing, and the sufferer may have difficulty swallowing, the combination of which can produce the trademark “foaming at the mouth”.
The fact is just because we can figure out what a "sickness" is doesn't mean we can destroy it. Nature comes at us in strange ways. Nature blended with science is deadly.
Ive said it before in other forums, but what the hell I'll restate it here. If there was to ever be a zombie, they would have to come from a mutation of a virus, or a genetic alteration of a virus or human genes. I read a National Geographic article about a year ago on the five possible causes for zombies. They listed variations of viruses, and mutations. Another thing they mentioned that seemed really interesting was the possibility of a mutation of a parasite. One example they gave was a mutation of a parasite that already exsists called toxoplasmosa gondii. This parasite lives in rats, but can only reproduce in the intestinal tract of cats because of a enzyme found there. The parasite attacks the rats brain causing psychosis. The rates are more aggressive, and will fight other rats and larger animals. They seemingly attack cats, and other predatory animals. Thus causing the rats to be eaten, and making their way into the intestines of cats.