"Every dead body that is not exterminated becomes one of them. It gets up and kills. The people it kills get up and kill."
"Undead" reffers to a corpse that has been animated in any way, and now walks the earth once more. This can be caused by a number of things, such as using rituals to temporalrilly re-amimate a corpse to carry out a task, or in some cases, victims of a fear, such as Toxicants in advanced stages.
Due to the nature of them being reanimated corpses, they are very fraigle, and easy to deal with. Contrary to popular belief, one does not need to destroy the brain of an undead in order to stop it (as they are still technically dead, they do not need their brains to live). One only needs render it unable to move, so dismembering it as much as possible, or burrying it under heavy objects is your best bet.