|The London Ontario Zombie Walk, 2011|
The zombie affliction only spreads where there is hope.
Picture a human survivor with no hope. She is attacked by 20 zombies, manages to kill 5, but then she gets bitten. Realizing she is fucked and has no chance to survive, she offs herself, or just lets herself get fully devoured by the zombies so she doesn't return to life. The 15 remaining zombies shuffle to the next survivor, who kills 5 before losing hope, rinse and repeat until there are no more zombies.
It's a quirk of (most) zombie mythology that zombies are not driven by reproduction. They just want to eat. Reproduction is a side effect of their meals being squirmy with hope, because it's only the humans who get away who become new zombies.
The people with hope are the bastards responsible for the spread. After getting bitten, they continue to fight for their lives. But really, after being bitten, they're just keeping a body fresh and nimble for a new zombie. That zombie is better able to find other suckers, ready to offer up their futile hopefulness as raw material for the next generation of the undead.
It's the human need for survival that ensures the spread of the affliction that will end all human survival.
P.S. Apply this metaphor to disease/consumerism/memes as you see fit.