Sept. 13, 2001 -- As television sets played and replayed the horrible events of Tuesday, Sept. 11, the whole world was watching. For most of us, it was more than watching -- it was a significant trauma.

Of course, affected most are those who survived the attacks on America from ground zero -- and their loved ones. But the shattering shock waves of this disaster radiate outward across the nation and even across the oceans.

"This is the beginning of the aftermath phase," stress disorder expert Mark I. Levy, MD, tells WebMD. "First was the unreality -- we feel this horror must be a bad dream, a feature film, a nightmare. But starting yesterday morning, people woke up and it hadn't gone away."

Most of us are having the same experience, the sense that this can't really be happening. Memories of the awful scenes leap unbidden into our minds. We may even try to avoid talking or even thinking about it. These are the symptoms of acute stress disorder -- a very real psychological condition, says Russell J. Kormann, PhD, associate director of the posttraumatic stress disorder program at Rutgers University's Anxiety Disorders Clinic. It differs from the long-lasting illness known as posttraumatic stress disorder (PTSD) only in that for most, it will pass -- but not for all of us.

"The problem of unreality is the first diagnostic symptom of an acute stress disorder -- that is what we are seeing right now," Kormann tells WebMD. "The unreality, or the denial that I think people may be experiencing right now is a psychological attempt to protect themselves against the horrific events of last Tuesday. It is identical to what you see with combat veterans. I think it is clear that PTSD is going to be a problem as time goes along."

Levy, an assistant clinical professor of psychiatry at the University of California, San Francisco, leads the community outreach arm of the San Francisco Psychoanalytic Institute. He says that even as far away from the Pentagon and World Trade Center as California -- and beyond -- people are showing signs of acute stress.

"We all feel more vulnerable and much less in control," he says. "We have an uneasy sense that the world has changed. There has been a loss of innocence we never will return to again. Our vulnerability has been brought home to us in spades. The basic phenomenon that results in PTSD is the rupture of a person's sense of invincibility. And now this has been true for virtually everyone."

This makes it likely to experience symptoms of acute stress: jumpiness, a short fuse, or impatience when we are driving. Or we may feel tearful, with waves of emotion suddenly washing over us. Instead of sleeping soundly, we may toss and turn, with disturbing thoughts swirling in our heads.

"I think millions of people are feeling this now, and it is important for us to know that," Levy says. "If you are coming to a speed bump in the road, you slow down. The more we know about the abnormal responses we will have, the more prepared we are and the less disruptive they are. People who won't ever get PTSD but who break down crying and have waves of emotion feel they are losing it. It is important for people to hear that this is normal. These are the predictable ways people have of responding to traumatic events."

Kormann, too, stresses the fact that these seemingly over-the-top emotions are normal reactions to terror.

"My mantra has been to give credit to the trauma," he says. "This is a horrific event. We have to give it its just due. If you are feeling full of distress, that makes some sense. Give people permission - give yourself permission -- to feel those feelings as sensible and not as an overreaction. And if you are having strong feelings, you are going to have to do what you may not want to do -- talk about it with others."

Levy says that in four to six weeks, a different reaction will set in. This is the sense that enough is enough, and that it is time to stop talking about it and to get on with our lives. We may even become irritated with those who don't feel this way -- but these may be the very people who are most in need of help.

"People who are traumatized should get professional counseling," he says. "If I were to screen a population of people for PTSD, I would look for people who can't stop talking, thinking, and reacting to this. They are in trouble at that point [4-6 weeks after the event]. They are not functioning well. They may have no sense of the future; they are seeing through a glass darkly, they are having these hypervigilant states where they are very jumpy. Everybody has a tendency to self-medicate with alcohol or drugs, but that is particularly true for those developing PTSD. It's OK for us to feel like we need to have a couple of stiff drinks, even though that won't help -- things that remove our inhibitions make us feel even more out of control. It is the wrong solution to the problem. But a lot of people with PTSD have dual diagnoses of alcoholism or addiction."

Washington University psychiatrist Carol S. North, MD, MPE, led a study of PTSD among survivors of the Oklahoma City bombing. She found that people who relied most on numbing and avoidance to deal with their trauma were more likely than others to develop PTSD.

Kormann and Levy note that people who have not fully resolved earlier traumas may be at greater risk of PTSD from the current trauma. But Kormann says he doesn't like to set a firm time line for when we should worry about a person developing PTSD.

"Individually we should look for people still struggling with dreams or inability to concentrate or irritability or having trouble with interpersonal relationships with their family or co-workers or bosses," he says. "The only time lines we as clinicians are really concerned about are anniversaries - a month or a year, that provides reference point. The PTSD symptoms are kind of re-energized by anything that is associated with the event."

This, Kormann notes, is where psychotherapy can help.

"Therapy takes it from something a person has to relive all the time to something that is experienced only as a sad memory," he says. "Then people with PTSD won't have to limit their lives by trying to avoid all reminders of the traumatic event."

Levy notes that new research points toward early treatment of people with acute stress disorders to prevent the development of chronic PTSD. He also notes that the trauma we all have just undergone makes us more susceptible to suffering from new traumas.

"If there is a subsequent event in our lives, our defensive structure may be shakier," he says. "If either personally or as a community we experience an overwhelming trauma in the future, we will be more shaken. The people who are affected by those events will be more vulnerable to PTSD because they have been though this event. It doesn't mean everybody will get PTSD."

Indeed, all of the experts who spoke with WebMD say that we'll always feel very, very sad about this week -- but the vast majority of us will be just fine.