who are the unconsecrated? what is the only way to kill the unconsecrated? And what cause it to be fast?

The unconsecrated were an aggressive, flesh-eating people, who were once dead. I only see evidence that the Unconsecrated need to be killed, not a mention of how. I see repeated evidence that it requires a weapon, and yet, in Chapter 28, Mary is prepared to kill one even without a weapon. I believe a quick death would be required in order to keep infection from spreading.