De juiste verklaring is dat iedereen het pathogeen (onbekend) in zich heeft en dat deze het immuunsysteem verlaagt.
Dus een beet van een zombie zorgt ervoor dat ze aan infectie van 'normale' bacterien sterven. Ook andere ziektes die je normaal gezien zou overleven (zoals de griep) kunnen dodelijk zijn. En nadien gaat het onbekende pathogeen je terug tot leven brengen.
http://walkingdead.wikia.com/wiki/Zombies
"The pathogen itself does not kill its hosts, but it seems to weaken their immune systems considerably, as even minor illnesses are far more fatal than normal to humans."
Infection
As previously stated, the zombie pathogen itself is not lethal, and zombie "infection" occurs due to high concentrations of antibiotic-resistant bacteria found in zombies, especially in their mouths. In the Walking Dead universe, every human being on the planet is believed to be infected, suggesting the disease is airborne or waterborne. Any time a human dies they will reanimate, bitten or not, unless their brain is destroyed. It is unknown if getting blood or saliva from zombies into human mouths will cause infection, but it is known that any contact zombie fluids make with open wounds, and possibly a human's eyes, will spread their lethal bacteria throughout the host's body.
Bites
Zombie bites are not necessarily fatal because of the zombie pathogen, but rather the unsanitary nature of their mouths due to diet and decomposition. Scratches cause similar infections for similar reasons.
It should be noted that zombie saliva and scratches are not guaranteed to kill, as both Tyreese and the Governor have come into contact with both and were unaffected. The deep, fatal infections left by zombie bites can potentially be avoided if the bite is on an appendage, which then must be amputated immediately after the bite. However, this does not always work, and bites on main parts of the body, or on veins or arteries, and always fatal.
