And he is by no means alone. In the past several years it has become evident that therapists, emergency personnel, the police and family members who deal with traumatized individuals can develop symptoms of PTSD secondhand. They endure what are called intrusions—images, flashbacks and nightmares that cause them to experience the horrible events over and over—even though the memories are not their own. Like people who have themselves been terrorized, they live in a state of stress-induced hyperarousal, with an overly active fight-or-flight response. They may suffer from sleep disorders and feel utterly hopeless.

The most recent edition of the Diagnostic and Statistical Manual of Mental Disorders acknowledges the problem. A diagnosis of PTSD no longer requires the immediate experience of a traumatic event; a person need not have been a victim or even an eyewitness. It is enough simply to hear the details. Recent research has begun to clarify how common the problem is and why some people are more susceptible to it than others.