At any given time approximately 10% of the adult population of the United States alone is suffering from clinical depression (http://www.clinical-depression-symptoms.com/anxiety-depression-statistics.htm: Mental Disorders in America. (2001, January 1). In "The Yellow Wallpaper" the woman is "diagnosed" with having stress and anxiety when in reality she has postpartum depression disorder - or so we can assume, it could truly be any form of depression. Misdiagnosing is all too common for people with a mental disorder, as is not even being diagnosed or treated at all. There are no truly factual numbers given for how many people go untreated per year but the National Institute of Mental Health estimates the numbers to be at or near 100 in every 1000 people having some form of clinical mental health illness which goes untreated. Each year approximately 30,000 people die from suicide, 90% of whom have a treatable or diagnosable mental health disorder (http://www.clinical-depression-symptoms.com/anxiety-depression-statistics.htm: Mental Disorders in America. (2001, January 1). One may remember that in "The Yellow Wallpaper" the lead woman also had a treatable and very diagnosable mental health disorder by today's standards, and she too is presumed to have commit suicide.
Nearly 14% of people in the United States are diagnosed each year with some form of anxiety which could include panic disorder, obsessive-compulsive disorder, post-traumatic stress disorder, generalized anxiety disorder, and phobias; social phobia, agoraphobia, and specific phobia (http://www.clinical-depression-symptoms.com/anxiety-depression-statistics.htm: Mental Disorders in America. (2001, January 1). In "The Yellow Wallpaper" it was said to be her nerves, or anxiety, and while this was part of her disease it was only a very small part, as postpartum depression disorder is now known to be a mix of anxiety disorders, mainly abandonment phobias and generalized anxiety.