The key to these questions is the emotional response we call anxiety. Unlike hunger or thirst, which build and dissipate (驱散) in the immediate present, anxiety is the sort of feeling that sneaks up on you from the day after tomorrow. It's supposed to keep you from feeling too safe. Without it, few of us would survive. All animals, especially the small, scurrying kind, appear to feel anxiety. Humans have felt it since the days they shared the planet with saber-toothed tigers. But we live in a particularly anxious age. The initial shock of Sept. 11 has worn off, and the fear has lifted, but millions of Americans continue to share a kind of generalized mass anxiety. A recent TIME/CNN poll found that eight months after the event, nearly two-thirds of Americans think about the terror attacks at least several times a week. And it doesn't take much for all the old fears to come rushing back. What was surprising about the recent drumbeat of terror warnings was how quickly it triggered the anxiety so many of us thought we had put behind us. This is one of the mysteries of anxiety. While it is a normal response to physical danger — and can be a useful tool for focusing the mind when there's a deadline looming (或隐或现) — anxiety becomes a problem when it persists too long beyond the immediate threat. Sometimes there's an obvious cause, as with the shell-shocked soldiers of World War I or the terror-scarred civilians of the World Trade Center collapse. Other times, we don't know why we can't stop worrying. There is certainly a lot of anxiety going around. Anxiety disorder — which is what health experts call any anxiety that persists to the point that it interferes with one's life — is the most common mental illness in the U.S. In its various forms, ranging from very specific phobias (恐惧症) to generalized anxiety disorder, it afflicts 19 million Americans. And yet, according to a survey published last January by researchers from UCLA, less than 25% of Americans with anxiety disorders receive any kind of treatment for their condition. 'If mental health is the stepchild of the health-care system,' says Jerilyn Ross, president of the Anxiety Disorders Association of America, 'then anxiety is the stepchild of the stepchild.' Sigmund Freud was fascinated with anxiety and recognized early on that there is more than one kind. He identified two major forms of anxiety: one more biological in nature and the other more dependent on psychological factors. Unfortunately, his followers were so obsessed with his ideas about sex drives and unresolved conflicts that studies of the physical basis of anxiety languished (减退). What can be the most suitable title of this passage?