The first hint is a sickly pig - in retrospect, signally the arrival of a swine flu, or something even more deadly. Given the wretched conditions in which humans are living in this zombie world, another plague coming along is not surprising.
And, given that everyone has been infected with the plague that turned so much humanity into walkers, and upon death any infected human will turn into a walker, the new plague has an especially perilous consequence: anyone who dies of this new plague will soon turn into a walker.
There may be a hopeful possibility here, though - and bear in mind that I haven't read the comics, so all I know of The Walking Dead is what I see on television - but I'm wondering if there's any chance that the new plague might kill not only humans and pigs, but walkers? I know, they're dead already, but they're nonetheless animated or mobile, and a new plague could in theory destroy a part of their body that moves them, drives them to feed on humans, etc. Just a thought.
In any case, there's no doubt that deaths from the new plague will put our community at all kinds of risk, as people they took in last year become infected and die. We get a nice nasty taste of what this could be like in the main action scene in the episode, in which Daryl, Michonne, Glenn and their party get rained on with walkers falling down on them through a crumbling roof. It's a dramatic example of walkers suddenly appearing in their mist.
Lots of good tinder for 4th season, which I'm much looking forward to.
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