Traumatic events don’t just affect the people who experience them. They also affect the victim’s partner, parents, children, and friends. We know this intuitively, but Scientific American highlights new research showing that the impact of trauma goes even deeper: Post-traumatic stress disorder (PTSD) might be passed from person to person.

By describing the traumatic event to another person, a form of secondary PTSD can be "caught" by someone who is close to the trauma victim, such as a parent, spouse, or even a therapist or emergency responder. According to Scientific American, recent research suggests that 10 to 20 percent of people who have a close relationship with someone who has PTSD could develop the condition themselves. One study from 2013 found that nearly one in five healthcare workers who had been helping members of the military with PTSD had developed “secondary trauma” [PDF].

Some of the symptoms they experienced included intrusions, or mental images, flashbacks, or nightmares of the traumatic event. Other symptoms were sleep disorders, feelings of hopelessness, stress-induced hyperarousal, and an overreactive fight-or-flight response.

Similar studies revealed that emergency responders, social workers, trauma therapists, and the wives of former prisoners of war are also at risk. Although the spouses or partners of war veterans are often affected, research from 2017 showed that the parents of veterans seemed unaffected, while the children of veterans occasionally showed symptoms, but not severe ones.

The definition of the disorder has even been amended to reflect these findings. According to the updated Diagnostic and Statistical Manual of Mental Disorders, firsthand experience of a traumatic incident isn’t necessary to be diagnosed with PTSD.

Psychologist Judith Daniels of the University of Groningen in the Netherlands suggests there’s a physiological explanation for why secondhand trauma can seem so real and vivid to someone who never experienced the trauma itself directly. “The regions of the brain that proce[ss] visual imagery have a very strong overlap with regions that process imagined visual experience,” she tells Scientific American. It would seem that just hearing about the traumatic event is enough to produce PTSD-like symptoms.

Researchers also found that extremely empathetic people and people who don’t keep any “emotional distance” from the trauma victim (such as spouses) are at greater risk of developing secondary PTSD. That’s partly because they may internalize the trauma.

There may also be a genetic aspect that allows PTSD to be passed down from parent to child. A 2017 study suggests that one’s genetic biomarkers could denote a higher risk of PTSD, but researchers said further studies are needed to identify the specific genes involved, CNN reports.

[h/t Scientific American]