Anxiety disorder often coexist with other mental health conditions such as depression or substance abuse problems. If not treated together, these conditions can make each other worse. Anxiety is the most common mental illness in the United States. They affect 40 million adults aged 18 and older in the United States, or 18% of the population each year. Anxiety disorders can start at any age, but they often start in childhood or adolescence. Anxiety is causeless. It can be triggered by a stressful situation, such as a job interview or a first date. It can also be caused by an event that has nothing to do with you or your life — for example, if you’re on vacation and hear explosions or gunshots outside your hotel room window.