They were killed, but they didn't die.They must be killed to death.
Realistically, the plague would make you a zombie, then you die.
Zombies would be considered "dead" since they're brainless and their senses would be null.Oh and, the reasons the zombie would bite people would be because the plague is advanced enough to make the zombies 'want' to spread the plague.
But how is this specifically sensed and confirmed by the disease?
Reminds me of AMC's The Walking Dead.