Fatal is used to describe anything that is capable of capable of causing or that actually has caused death; it carries a strong suggestion of the inevitability of fate: an illness which might not be serious for a young person, but which will almost certainly prove fatal to the old lady. Deadly is interchangeable with fatal in this sense: Leukaemia is a deadly disease. Deadly , however, in a way that fatal cannot, can refer to a person who desires or seek to cause the death of another person: The murdered man had many deadly enemies. Mortal , like deadly , can be applied both to things or people that cause, are capable of causing, or seek to cause someone’s death: Because of an ancient family feud, the two cousins had been mortal enemies from birth. In its reference to things, mortal differs from fatal and deadly only in the fact that it is usually found in contexts which detail a death that has already occurred: He was struck down by a mortal blow upon the head. Lethal refers to something which, because of some intrinsic quality in its make-up is certain to cause death, and may indeed exist for the express purpose of causing death: Cyanide is a lethal poison.

With the exception of lethal , all these words can be used to describe something that causes great fear or discomfort, or that brings about disaster or ruin, but that does not lead to physical death: a fatal mistake; a deadly insult; in mortal terror.