Zombie origins: How will the ZA gegin? Who knows. Luckily we have had movies, books, comics, videogames, ect. to help us figure it out.
There could be a plague of some sort that will cause the breakout that willl lead us to zombie takeover. a virus (think resident evil t-virus), a mutated already exsiting pathogen (think quarantine and the new form of rabies that turn infected people into crazied zombies), or a parasite (think The Dicrocoelium dendriticum, yes this is a real parasite, google it. did you google it? scary right? image it mutating to be able to control humans.)
One of the major causes of bringing on the ZA is govt experimentation. Just like in all the return of the living dead movies, that gov could bring about the Z-day. How did 28 days later begin? when scientist were genectically altering "rage".
There are also a possibily of alien forces at hand, think night of the comet or night of the creeps. Both ZA are caused by alien intervention.
There could also be a demon virus that will lead to the "possessed" biting and creating zombies. the only ones who will be able to save us in the vatican. Think REC (Quarantine is the american remake)
no matter the cause the result is still the same. you get bit, you get infected. so dont get bit!
But what does matter is how it can be transmitted besides being bit. the walking dead, we are already infected. we just dont become zombies until we die. 28 days later, we can die and know we wont become like one of those crazied ragers. shaun of the dead tells us even if we become a zombie we can still have a happy life, if we have someone to lock us in the shed and play videogames with us, like a best friend.