3 Reasons Why A Zombie Apocalypse Is Possible

• Parasites- A parasite's main function once it attaches to something that could be a host is self preservation. It will take over the body to preserve itself and then transform it to feed as it needs as well as provide the host with abilities, antibodies, or skills to function as it does so that it can live longer. As long as the parasites are maintained, they will live a long and healthy life. A possible parasite that is easy to acquire comes from cat feces called Taxoplasma.
• Viruses- Like in the movie Contagion, (kind of), a virus or other illness can spread quickly. It doesn't take much for a disease to transfer from person to person, especially if you don't know you have a virus. Once it starts it will be difficult, if impossible, to stop because it will take control quickly and spread quicker-- especially if it really does feed on brains.
• Animals- Animals contain a completely different set of illnesses and diseases, most of which humans cannot get. However, there are a few that can be passed from animal to human such as rabies. A transference from an animal could bring on animalistic qualities that would make humans hunt each other.