mental illness vs neurasthenia

mental illness

noun
  • A mental disorder. 

  • The property of being mentally ill; mental disorders taken as a whole. 

neurasthenia

noun
  • An ill-defined medical condition characterized by lassitude, fatigue, headache, and irritability, associated chiefly with emotional disturbance. 

How often have the words mental illness and neurasthenia occurred in a corpus of books? (source: Google Ngram Viewer )