The afflicted are victims of the Sanguinesti Affliction, a disease that infected Mort'ton in early Fifth Age. The disease itself spread to the village from the neighboring Sanguinesti region, although it is unclear how. The afflicted have a very limited mental capacity, and are highly aggressive to non-infected individuals. They speak nonsense if approached and have a sickly green colour, as well as a disoriented way of walking.
The Afflicted may be cured temporarily by either of the two serums, Serum 207 or Serum 208. After curing the afflicted, the cured villager(s) will become a Mort'ton local and will give rewards of various types, many related to making the cures. The Serum 208 yields better rewards from villagers.
Serum 208 is supposed to be a permanent cure for the Sanguinesti affliction. However, if used on an afflicted villager, it only cures them temporarily, probably to avoid making the serum useless when all the afflicted are cured.
Despite them supposedly being aggressive to non-afflicted people, the afflicted in-game are not aggressive.