For a medical or other condition to be a "disability" covered by the Americans With Disabilities Act (ADA) two basic things are necessary: the condition must be (1) substantially limit the for the person (2) a major life activity.
Here's an example from a recent case, Jacobs v North Carolina Administrative Office of Courts. Jacobs worked as a deputy courthouse clerk at a county courthouse in North Carolina. She was diagnosed with and had long suffered from a condition known as Social Anxiety Disorder. She requested a reasonable accommodation, to be assigned to work elsewhere in the clerk's office aside from the front desk where she was called upon to interact with lawyers, defendants, and other members of the public throughout the day. Her employer refused and instead made up a reason to fire her. Jacobs filed suit claiming that the refusal to reassign her and her firing violated the ADA.
The court first considered whether Jacobs' condition affected a "major life activity." This was an easy question, since a "major life activity" is one of "central importance to daily life" and "few activities are more central to the human condition than interacting with others."
Then the employer argued that Jacobs was not substantially limited in interacting with others because she “interact[ed] with others on a daily basis,” “routinely answered inquiries from the public at the front counter,” “socialized with her co-workers outside of work,” and engaged in social interaction on Facebook. This argument when nowhere and too far as the Court observed:
The [EEOC] regulations define a substantially limiting impairment as one that “substantially limits the ability of an individual to perform a major life activity as compared to most people in the general population.” “An impairment need not prevent, or significantly or severely restrict, the individual from performing a major life activity in order to be considered substantially limiting.” ... A person need not live as a hermit in order to be “substantially limited” in interacting with others.
The key here, the main test is whether the condition substantially limits the individual's ability to do the activity "as compared to most people in the general population." I'd put it, does the condition make it harder for the individual than it does for most people in the general population?