The screams of a girl being stabbed in her own high school haunted Karen Perrin last night.

The Langley, B.C., grandmother wasn't inside Abbotsford Senior Secondary when yesterday's fatal stabbing happened, and didn't hear about it from anyone who was.

Instead, she saw the video online, before realizing what she'd clicked on.

"At first I thought it was a guy helping her … then I saw him throw the knife and realized I'm actually watching a horrific video of this poor young lady being stabbed," said Perrin.

That video, filmed on a cellphone from inside the school, has gone viral — a common shorthand for whatever spreads rapidly online.

But the violence seen here is a virus of a different sort, say psychologists, a source of trauma not just for the victims and their families, but able to infect anyone who watches.

'Trigger to trauma'

Police and the school district have tried to stop the video's spread, exhorting the media and others not to seek, watch or share it.

"Out of respect for the families, I ask that you immediately cease circulation of the viral video that was filmed during this violent incident yesterday," said Abbotsford school superintendent Kevin Godden today.

Abbotsford School District superintendent Kevin Godden, along with Abbotsford police and homicide investigators, is imploring the public not to share the video. (Rafferty Baker/CBC) Many media outlets, including CBC News, are not posting or broadcasting the video.

"This video is a trigger to trauma, not only for our students and our community, but for any person that has been involved in a traumatic incident," said Godden.

That's not an exaggeration, but a reality of our digital age, say experts.

"It's still very powerful information to see something on video, even if it isn't live, knowing that the outcome was unfortunately the death of a teenager," said Jennifer Shapka, a UBC professor in the department of educational and counselling psychology.

Shapka, who studies cyberbullying, said work in that field shows a digital experience can cause more damage than face-to-face contact.

"That experience is permanent once you put it online, and stays in that original, most potent form every time you see it," said Shapka.

And for teens, the video may have followed them home — on their phones, their social media feeds, even into their bedrooms.

"That event is there 24/7, enhancing the trauma and emotional distress."

Flags were at half-mast and the school was closed on Wednesday as police continued the homicide investigation. (Rafferty Baker/CBC)

Oriented to threat

There is a reason, beyond base voyeurism, why people are naturally curious about terrible things that happen to others, said Gina Vanderham, a registered clinical counsellor in Vancouver.

At an animal level, in the wiring of our brains, "we're oriented to threat," she said.

"We go, 'Oh my God, this happened at the school … what if it happened to us? How are we going to prepare?'"

Those instincts don't make watching a good idea, Vanderham cautions.

"Simply watching the video could bring up symptoms of stress and trauma," depending on how vulnerable someone is, or their past history of witnessing or experiencing violence, she said.

Hypervigilance, getting angry, freezing up and flashbacks from the event can all be acute stress reactions, said Vanderham.

People may shake with nervous energy, or try to dull the feelings with drugs and alcohol.

In the case of children, there's enough research on aggression and violence to say there's no educational value in the video, said Shapka.

"There is no good that can come from traumatizing a child to show them a video of this sort."

The memorial outside the school grew throughout the day on Wednesday, with candles, flowers, and stuffed animals. (Tanya Fletcher/CBC)

'You can't retract the internet'

We might wish the internet had some giant undo button, but that's not how it works.

"You can't retract the internet. The internet has it," said Jesse Miller, a social media consultant in North Vancouver who does workshops for schools.

What can be done, is to have conversations with children at an earlier age about how to handle their impulse to document and share their lives.

"What happens when you do have video, when you do have information on your device and you feel like you need to share it?" says Miller.

"Can you take 10 seconds to ask yourself, 'What will the impacts be?'"

Beyond helping children cope with anxiety or fear after the stabbing, parents can help their kids "see through the screen" to imagine who their actions will touch, said Shapka.

"It's not just a video that's been developed for your entertainment. This is a real person that it has happened to."