"Anna O." was the pseudonym for Bertha Pappenheim, a patient of the physician Josef Breuer in the late 19th century. She is often considered one of the first cases of psychoanalysis, as her treatment involved talking about her symptoms and experiences. This approach laid the groundwork for the development of Sigmund Freud's theories on the unconscious mind and the importance of talking therapy. Her case highlighted the significance of psychological factors in physical ailments, leading to the concept of "hysteria." Anna O.'s experiences and treatment contributed to the understanding of mental health and the evolution of psychotherapy, influencing future practices in the field.