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Pathogen
Most, if not everyone, on the planet within The Walking Dead universe somehow contracted the pathogen that, for reasons and through means unknown, brings the dead back to "life." It is unknown how the disease is spread, though its almost-total infection rate worldwide suggests it is either water-borne, air-borne, or both. The exact taxonomy of the pathogen is unknown. The pathogen itself does not kill its hosts, but it seems to weaken their immune systems considerably, to the point where even minor illnesses are far more fatal than normal to humans.