A zombie is a former living human who has been infected with the virus called Solanum. While this particular virus is not airborne, Solanum is 100 percent communicable through direct fluidic contact with the infected. It is also 100 percent fatal. Once the Solanum virus enters the bloodstream of the living, it travels throughout the body, systematically shutting down all bodily functions which, of course, terminates the life of those infected. During this process, it also mutates the human brain, transforming it into another organ altogether. Once the metamorphosis of the brain is complete, it is able to reanimate and control the corpse, turning it into a member of the walking dead.
Resource: The Zombie Survival Guide