In The Walking Dead it's never actually revealed what started the scourge of Walkers that plague the earth or who or what was patient zero. In many zombie fictions it tends to be a virus or disease which began in a lab or some far flung jungle here on earth.
In a recent episode of PBS Space Time Gabriel Perez-Giz explores the idea of the zombie apocalypse starting in space. It might sound ridiculous, but space can have a strange effect on viruses and bacteria, sometimes turning them far deadlier. Prepare to be afraid.
Perez-Giz uses the example of an experiment conducted in space in 2006 with salmonella, where space was shown to make the bacteria a lot more lethal.
And it's not just bacteria. But viruses too might be altered in zero gravity, research points to it being highly likely.
Add to these the idea that quarantine and sterilisation procedures might not be completely perfect—and that in the future more people, both government and privately sent, will be heading to the cosmos, and the chances of an altered virus or disease starting a zombie apocalypse might not be so far-fetched.
Or, at least, might be a nice plot for George A. Romero to explore in a space zombie movie he hasn't yet made.