According to 'The Zombie Survival Guide' by Max Brooks. The virus started in Asia. Remember all those pictures of the Chinese and Japanese wearing the dust masks.
The Media said it was because of the 'Bird Flu' or Avian Flu Virus. That was an attempt to keep the masses from Panicing.
Then like in the Stephen King story, 'The Stand.' The virus spread Rapidly.
There are other Theories. But this is the one that makes the most since to me.like