Ed Grabianowski suggests how a zombie outbreak could (semi-plausibly) happen:
In virtually every zombie scenario, zombies are able to function despite increasing levels of physical deterioration due to injury or decomposition. There has to be some mechanism for transmitting neural impulses from the brain to various body parts, and for providing energy to muscles so they can keep operating.
The most common science fictional explanation for zombie outbreaks is a virus — but viruses and bacterial infections are not known for building large new physical structures within the body. So let’s count viruses out. Instead, the need for a mechanism to activate deteriorating body parts actually provides the cornerstone of what is, in my opinion, the strongest theory: fungal infection.
We know that fungi can infect humans. We also know that fungal networks exist in most of the world’s forests. These mycorrhizal networks have a symbiotic relationship with trees and other plants in the forest, exchanging nutrients for mutual benefit. These networks can be quite large, and there are studies that demonstrate the potential for chemical signals to be transmitted from one plant to another via the mycorrhizal network. That, in turn, means that fungal filaments could perform both vascular and neural functions within a corpse.
This leads us to the following scenario: microscopic spores are inhaled, ingested, or transmitted via zombie bite. The spores are eventually dispersed throughout the body via the bloodstream. Then they lie dormant. When the host dies, chemical signals (or, more accurately, the absence of chemical signals) within the body that occur upon death trigger the spores to activate, and begin growing. The ensuing fungal network carries nutrients to muscles in the absence of respiration or normal metabolism.
Part of the fungal network grows within the brain, where it interfaces with the medulla and cerebellum, as well as parts of the brain involving vision, hearing and possibly scent. Chemicals released by the fungi activate basic responses within these brain areas. The fungi/brain interface is able to convert the electrochemical signals of neurons into chemical signals that can be transmitted along the fungal network that extends through much of the body. This signal method is slow and imperfect, which results in the uncoordinated movements of zombies. And this reliance on the host’s brain accounts for the “headshot” phenomenon, in which grievous wounds to the brain or spine seem to render zombies fully inert.