This vicarious or secondary traumatic stress is relatively rare, but "we've all identified it, " he said.

CNN: Treating trauma victims may cause its own trauma

"There is a second territory ... and that is secondary traumatic stress injury, " says Shay, the author of Odysseus in America and Achilles in Vietnam.

NPR: Chaplain Struggles with PTSD from Time in Iraq

The symptoms of vicarious trauma, also called secondary traumatic stress, parallel those of direct trauma, said Laurie Anne Pearlman, former president of the Trauma Research, Education, and Training Institute Inc. and author of several books about vicarious trauma.

CNN: Treating trauma victims may cause its own trauma