Small town, it has covered with clouds. That is a virus, that is an epidemic. I do not remember, people like turning into a zombie . People trying to get out of this town, but can not - around a huge wall of thorns. Another point: in this case, like the aliens are involved, sometimes pouring acid rain, somebody got a a small plane, fly it, make their way through the clouds and then hang in the air the inhabitants of this town.