Explanation:

- Fatal: This term means deadly or leading to death. It's appropriate here because the illness being described is severe and potentially life-threatening.

- Suspicious: This suggests something that arouses suspicion or distrust. It doesn’t fit well with describing the nature of an illness.

- Desperate: This means feeling a situation is so bad as to be impossible to deal with, which doesn't match the context of illness severity.

- Dreary: It refers to something that is dull or boring, not applicable to describe an illness.