Zombie Genesis

Exactly how did the zombie outbreak occur?  Well that depends on who's telling the story but generally it has to do with a reanimation virus.  In the 1968 Night of the Living Dead, it was speculated that a space probe entering earths atmosphere exploded and released radioactive contamination, thus reanimating recently deceased human beings.

Now if we are talking Resident Evil zombies, they were the result of an accident at an Umbrella research facility.  A sample known as the T-virus broke and was release into the air.  Long story short, everyone in the facility ended up dead and the T-virus reanimated their corpses.  The same principals apply, dead body walking around trying to eat you.

In the 2002 British flick 28 Days Later, some animal rights activist decide to free some chimps from a research facility against the warning of the chief scientist who warns them that they are infected with a "Rage" virus.  Monkey attack...people get infected...  In most every zombie flick there is a delay from the time you are bitten to when you actually turn into a zombie however , not with the "Rage" virus.  The turn around time is almost instantaneous and these aren't your average zombies that just wander around aimlessly tripping over themselves like a new baby deer, these bastards are quick.
In the 2007 film, Flight of the Living Dead: Outbreak on a Plane, a scientist infected with a genetically engineered virus is smuggled on to a 747 in a container. The virus he is infect with was designed to make soldiers able to continue to fight after being mortally wounded and also gives them superhuman strength. The outbreak starts when the plane goes through some turbulence and the container get damaged releasing the infected scientist. As you can imagine he starts chewing on passengers.
In the 2009 comedy, Zombieland, the plague is started by a mutated strain of mad cow disease.
In the current TV series The Walking Dead, it would appear that the virus is spread fairly easily like the flu or a common cold since everyone is infected.  You don't have to be bitten in order to turn into a zombie, you simply just have to die and you become reanimated within minutes to hours later.  I won't go into further detail on how this outbreak got started since this is still an ongoing show and it hasn't been revealed yet.
I could go on about how the zombie infections start in every movie I've seen but you get the point. It starts either by radiation or most commonly by some virus. Regardless of how it starts all outbreaks have one thing in common, if you become one there is no cure.