Zack Snyder's 2004 remake of Dawn of the Dead operates on different rules than the original, causing how the oubreak spread to become a plot hole. Horror remakes generally get a bad rap from the genre community, and for the most part, that's understandable. There are, after all, lots of really, really bad remakes and reboots of classic horror movies, some of which are so bad they can actually tarnish the name of the original.
Yet, it bears repeating that not all horror remakes turn out bad. In fact, some end up eclipsing the reputation of the film they remade, such as David Cronenberg's The Fly and John Carpenter's The Thing. To be sure, Snyder's Dawn of the Dead remake isn't one of those films, as very few would argue it betters or even equals the 1978 classic from zombie master George A. Romero. Still, Dawn of the Dead 2004 is a really good film in its own right, with many who went into it dreading a remake of Romero's undead epic coming out pleasantly surprised.
Sure, zombie purists - including Romero himself - aren't fans of the way the zombies run like Olympic sprinters in Snyder's remake, but that aside, opinion has always been kind to the film. However, it's not perfect, and that includes the script, which fails to explain a major plot point.
Dawn of the Dead 2004's Zombie Outbreak Is a Huge Plot Hole
While George Romero's zombie films never conclusively explained the source of the initial zombie uprising, it did make clear that all the recently deceased with intact brains suddenly returned as undead monsters, and that anyone who died from then on would also rise if their brain wasn't destroyed beforehand. However, in Snyder's Dawn of the Dead - written by future Guardians of the Galaxy director James Gunn - it's specified that only those bitten by a zombie will die and be resurrected as a zombie. If one were to die without being bitten, they'd stay dead, because this zombie plague spreads like a virus.
The problem there is, if it takes a zombie to make a zombie in Dawn of the Dead 2004, how exactly did the contagion spread in the first place? Who was the first zombie, and if so, how did they get that way? In interviews after the fact, Snyder suggested the zombie plague was of supernatural origin, and transmitted via bites like with a vampire or a werewolf, but this is mentioned nowhere in the film proper. Is the zombie virus man-made? If it's supernatural, wouldn't that mean there has to be some kind of "master" zombie out there somewhere that gave the first bite? If so, how did they become a zombie? Every attempt to solve this big plot hole just leads to more questions that eventually just fold in on each other. At that point, fans might as well throw up their hands and say that a wizard did it.