In Zombieland, the virus was actually mutated Mad Cow Disease that a human caught from eating a contaminated hamburger.
In 28 Days Later, a chimp infected with a virus meant to cause extreme aggression bit a human who turned almost immediately.
In I Am Legend (the remake), a cure for cancer mutated and infected every human on the planet. Most of the population died, but a portion was immune and a larger portion turned into the walking undead. Technically, these are classified as vampires by some, but the increased aggression and intense desire to consume human flesh are the two main criteria for defining a zombie, so I’ll classify these monsters as zombies as well.
In an episode of Sliders, the sliders encounter a world where most of the population has become zombie-like again, intense aggression and the desire to attack/eat humans being the defining characteristics and they became that way because of a new diet drug that “ate” fat.
In the book Xombies, the plague was engineered and infected women first. I don’t remember if the author mentions why the plague was engineered the first place but it’s an interesting read and I suggest reading it. These zombies are also very strange in that they are sentient and impossible to kill. Once could say that the sentience of these zombies has moved into their cells, so even when a zombie is destroyed completely, that is to say, broken down into nothing but cells, it is still self-aware and can still infect humans, but really only if ingested at that point.