By becoming an insect, Gregor crosses an imaginary line to a point where there is no turning back, just like any person suffering from a chronic illness. Kafka suffered from various illnesses throughout his life which contributed to his health-conscious vegetarian diet. He suffered from insomnia, recurring cough, night sweats and similar difficulties, all symptoms of tuberculosis. In 1914 there was no adequate technique for diagnosing tuberculosis, and the doctor identified Kafka's disease as bronchitis. He spent much of his time during these symptomatic attacks in a sanatorium with only his diaries for company. With his illness and his isolation Kafka felt like a parasite, unwanted, insulted. Kafka demonstrates this in his unpublished "Letter to the Father", where he defines himself as "Ungeziefer", which actually means worm. This shows that Kafka used Gregor as a puppet to express the burden he felt he was in his work, The Metamorphosis. Clearly, Franz Kafka used The Metamorphosis as a tool to express dominant themes that were present in his life. These themes include that of the father-son relationship shared between Gregor and Mr. Samsa/Franz and Hermann Kafka, the theme of isolation, and the theme of illness. These themes expressed the circumstances in vivid detail to communicate the personal pain that Franz felt as if he could not share it with anyone