* One theory is that the zombie outbreak was caused by a virus. This is the most common theory, and it is supported by the fact that the zombies in The Walking Dead seem to be infected with some kind of disease.
* Another theory is that the zombie outbreak was caused by a chemical spill. This theory is less common, but it is supported by the fact that the zombie outbreak in The Walking Dead seems to have started in a chemical plant.
* Finally, some fans believe that the zombie outbreak was caused by a supernatural event. This theory is the least common, but it is supported by the fact that the zombies in The Walking Dead seem to have some kind of supernatural power.
Ultimately, the cause of the zombie apocalypse in The Walking Dead is left up to the viewer's interpretation. However, the show's writers have said that they wanted the zombie outbreak to be a metaphor for the real-world threat of disease and contagion.