But in about one in three million doses, the live oral vaccine used in poor countries can mutate back into a wild-type virus that can infect and paralyze victims.

They usually infect victims that make skin contact with the water.

Finding shelter at the now abandoned gas station, they fend off larger incarnations of a parasite as it infects more victims.

But if one transmitted it to the other, "it would be very worrisome," he said, because previous resistant strains had been very weak at infecting new victims.

The deuteronomists somehow realized this and exterminated her by blocking all the vectors by which she infected new victims.

The first person, whom the credits call the Thief (played by Thiago Martins), infects victims with the worm to brainwash them and steal their money.