Post-traumatic stress disorder has been on psychiatry's books for just 23 years, and before the terrorist attacks of 9/11, no one thought watching TV images of trauma could give rise to the disorder.

The notion remains controversial, but new research suggests PTSD might be transmitted over the airwaves. A study in the journal PNAS found those who spent more than six hours a day watching media coverage of the April 15 Boston Marathon bombing and its aftermath suffered stronger stress reactions than did people who were directly involved but who watched less news coverage of the events.

Two to four weeks after the bombings, researchers reached out to 4,675 Americans to gauge their lingering stress reactions and their media exposure.

As much as a month after the bombings, about 4.5 percent of respondents were reporting symptoms that met the psychiatric criteria for "high acute stress." Respondents with a history of mental-health problems and those who had watched six or more hours of daily media coverage of the bombings were most likely to report high acute-stress symptoms. - L.A. Times