Depends on what you consider a zombie. More and more zombies are living people who get infected with some disease and turn. In 28 Days it was a rage virus that caused infected people to go into a fit of rage so bad that they attacked any and everyone. In WWZ it was a rabies virus that mutated DNA to turn people into a new species all together (at least that’s how it seemed). In the walking dead it’s a gene we all have in us that just flips on when we die. So it is plausible for a zombie apocalypse to happen if you think along the lines of 28 Days.